|Title||Email Greylisting: An Applicability Statement for SMTP
Kucherawy, D. Crocker
Internet Engineering Task Force (IETF) M. Kucherawy
Request for Comments: 6647 Cloudmark
Category: Standards Track D. Crocker
ISSN: 2070-1721 Brandenburg InternetWorking
Email Greylisting: An Applicability Statement for SMTP
This document describes the art of email greylisting, the practice of
providing temporarily degraded service to unknown email clients as an
Greylisting is an established mechanism deemed essential to the
repertoire of current anti-abuse email filtering systems.
Status of This Memo
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(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
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Table of Contents
1. Introduction ....................................................3
1.1. Background .................................................3
1.2. Definitions ................................................4
2. Types of Greylisting ............................................4
2.1. Connection-Level Greylisting ...............................4
2.2. SMTP HELO/EHLO Greylisting .................................5
2.3. SMTP MAIL Greylisting ......................................5
2.4. SMTP RCPT Greylisting ......................................5
2.5. SMTP DATA Greylisting ......................................6
2.6. Additional Heuristics ......................................7
2.7. Exceptions .................................................7
3. Benefits and Costs ..............................................8
4. Unintended Consequences .........................................9
4.1. Unintended Mail Delivery Failures ..........................9
4.2. Unintended SMTP Client Failures ...........................10
4.3. Address Space Saturation ..................................11
5. Recommendations ................................................12
6. Measuring Effectiveness ........................................13
7. IPv6 Applicability .............................................14
8. Security Considerations ........................................14
8.1. Trade-Offs ................................................14
8.2. Database ..................................................14
9. References .....................................................15
9.1. Normative References ......................................15
9.2. Informative References ....................................15
Appendix A. Acknowledgments ......................................17
Preferred techniques for handling email abuse explicitly identify
good actors and bad actors, giving each significantly different
service quality. In some cases, an actor does not have a known
reputation; this can justify providing degraded service, until there
is a basis for providing better service. This latter approach is
known as "greylisting". Broadly, the term refers to any degradation
of service for an unknown or suspect source, over a period of time
(typically measured in minutes or a small number of hours). The
narrow use of the term refers to generation of an SMTP temporary
failure reply code for traffic from such sources. There are diverse
implementations of this basic concept and predictably, therefore,
some blurred terminology.
Absent a perfect abuse-detection mechanism that incurs no cost, the
current requirement is for an array of techniques to be used by each
filtering system. They range in cost, effectiveness, and types of
abuse techniques they target.
Greylisting happens to be a technique that is cheap and early (in
terms of its application in the SMTP sequence) and surprisingly
remains useful. Some spamware does indeed route around this
technique, but much does not.
The firehose of spam over the Internet represents a wide range of
sophistication. Greylisting is useful for removing a large amount of
This memo documents common greylisting techniques and discusses their
benefits and costs. It also defines terminology to enable clear
distinction and discussion of these techniques.
There is some confusion in the industry that conflates greylisting
with an SMTP temporary failure for any reason. The purpose of this
memo is also to dispel such confusion.
For many years, large amounts of spam have been sent through purpose-
built software, or "spamware", that supports only a constrained
version of SMTP. In particular, such software does not perform
retransmission attempts after receiving an SMTP temporary failure.
That is, if the spamware cannot deliver a message, it just goes on to
the next address in its list since, in spamming, volume counts for
far more than reliability. Greylisting exploits this by rejecting
mail from unfamiliar sources with a "transient (soft) fail" (4xx)
[SMTP] error code. Another application of greylisting is to delay
mail from newly seen IP addresses on the theory that, if it's a spam
source, then by the time it retries, it will appear in a list of
sources to be filtered, and the mail will not be accepted.
Early references for greylisting descriptions and implementations can
be found at [SAUCE] and [PUREMAGIC].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [KEYWORDS].
1.2.2. Email Architecture Terminology
Readers need to be familiar with the material and terminology
discussed in [MAIL], [EMAIL-ARCH], and [SMTP].
2. Types of Greylisting
Greylisting is primarily performed at some phase during an SMTP
session. A set of attributes about the client-side SMTP server are
used for assessing whether to perform greylisting. At its simplest,
the attribute is the IP address of the client, and the assessment is
whether it has previously connected recently. More elaborate
attribute combinations and more sophisticated assessments can be
performed. The following discussion covers the most common
combinations and relies on knowledge of [SMTP], its commands, and the
distinction between envelope and content.
2.1. Connection-Level Greylisting
Connection-level greylisting decides whether to accept the TCP
connection from a "new" [SMTP] client. At this point in the
communication between the client and the server, the only information
known to the receiving server is the incoming IP address. This, of
course, is often (but not always) translatable into a host name.
The typical application of greylisting here is to keep a record of
SMTP client IP addresses and/or host names (collectively, "sources")
that have been seen. Such a database acts as a cache of known
senders and might or might not expire records after some period. If
the source is not in the database, or the record of the source has
not reached some required minimum age (such as 30 minutes since the
initial connection attempt), the server does one of the following,
inviting a later retry:
o returns a 421 SMTP reply and closes the connection, or
o returns a different 4yz SMTP reply to all further commands in this
A useful variant of the basic known/unknown policy is to limit
greylisting to those addresses that are on some list of IP addresses
known to be affiliated with bad actors. Whereas the simpler policy
affects all new connections, including those from good actors, the
constrained policy applies greylisting actions only to sites that
already have a negative reputation.
2.2. SMTP HELO/EHLO Greylisting
HELO/EHLO greylisting refers to the first command verb in an SMTP
session. It includes a single, required parameter that is supposed
to contain the client's fully qualified host name or its literal IP
Greylisting implemented at this phase retains a record of sources
coupled with HELO/EHLO parameters. It returns 4yz SMTP replies to
all commands until the end of the SMTP session if that tuple has not
previously been recorded or if the record exists but has not reached
some configured minimum age.
2.3. SMTP MAIL Greylisting
MAIL command greylisting refers to the command verb in an SMTP
session that initiates a new transaction. It includes at least one
required parameter that indicates the return email address
(RFC5321.MailFrom) of the message being relayed from the client to
Greylisting implemented at this phase retains a record of sources
coupled with return email addresses. It returns 4yz SMTP replies to
all commands for the remainder of the SMTP session if that tuple has
not previously been recorded or if the record exists but has not met
some configured minimum age.
2.4. SMTP RCPT Greylisting
RCPT greylisting refers to the command verb in an SMTP session that
specifies intended recipients of an email transaction. It includes
at least one required parameter that indicates the email address of
an intended recipient of the message being relayed from the client to
Greylisting implemented at this phase retains a record of tuples that
combines the provided recipient address with any combination of the
o the source, as described above;
o the return email address; and
o the other recipient addresses of the message (if any).
If the selected tuple is not found in the database, or if the record
is present but has not reached some configured minimum age, the
greylisting Mail Transfer Agent (MTA) [EMAIL-ARCH] returns 4yz SMTP
replies to all commands for the remainder of the SMTP session.
Note that often a match on a tuple involving the first valid RCPT is
sufficient to identify a retry correctly, and further checks can be
2.5. SMTP DATA Greylisting
DATA greylisting refers to the command verb in an SMTP session that
transmits the actual message content, as opposed to its envelope
This type of greylisting can be performed at two places in the SMTP
1. on receipt of the DATA command, because at that point the entire
envelope has been received (i.e., all MAIL and RCPT commands have
been issued); or
2. on completion of the DATA command, i.e., after the "." that
terminates transmission of the message body, since at that point
a digest or other analysis of the message could be performed.
Some implementations do filtering here because there are clients that
don't bother checking SMTP reply codes to commands other than DATA.
Hence, it can be useful to add greylisting capability at that point
in an SMTP session.
Numerous greylisting policies are possible at this point. All of
them retain a record of tuples that combine the various parts of the
SMTP transaction in some combination, including:
o the source, as described above;
o the return email address;
o the recipients of the message, as a set or individually;
o identifiers in the message header, such as the contents of the
RFC5322.From or RFC5322.To fields;
o other prominent parts of the content, such as the RFC5322.Subject
o a digest of some or all of the message content, as a test for
o analysis of arbitrary portions of the message body.
(The last four items in the list above are only possible at the end
of DATA, not on receipt of the DATA command.)
If the selected tuple is not found in the database, or if the record
exists but has not reached some configured minimum age, the
greylisting MTA returns 4yz SMTP replies to all commands for the
remainder of the SMTP session.
2.6. Additional Heuristics
Since greylisting seeks to target spam senders, it follows that being
able to identify spamware within the SMTP context beyond the simple
notion of "not seen before" would be desirable. A more targeted
approach might also include in its selection heuristics such as the
o If a DNS blacklist [DNSBL] lists an IP address but the implementer
wishes to be cautious with mitigation actions rather than blocking
traffic from the IP address outright, then subject it to
o If the value found in a PTR record follows common naming patterns
for dynamic IP addresses, then subject it to greylisting.
Most greylisting systems provide for an exception mechanism, allowing
one to specify IP addresses, IP address Classless Inter-Domain
Routing (CIDR) [CIDR] blocks, host names, or domain names that are
exempt from greylisting checks and thus whose SMTP client sessions
are not subject to such interference.
Likely candidates to be excepted from greylisting include those known
not to retry according to a pattern that will be observed as
legitimate and those that send so rarely that they will age out of
the database. In both cases, the excepted source is known not to be
an abusive one by the site implementing greylisting. Otherwise,
typical non-abusive senders will enter the exception list on the
first proper retry and remain there permanently.
One could also use a [DNSBL] that lists known good hosts as a
greylisting exception set.
3. Benefits and Costs
The most obvious benefit with any of the above techniques is that
spamware generally does not retry and is therefore less likely to
succeed, absent a record of a previous delivery attempts.
The most obvious detriment to implementing greylisting is the
imposition of delay on legitimate mail. Some popular MTAs do not
retry failed delivery attempts for an hour or more, which can cause
expensive delays when delivery of mail is time critical. Worse, some
legitimate MTAs do not retry at all. (Note, however, that non-
retrying clients are not fully SMTP-capable, per Section 2.1 of
[SMTP]. A client does not know, nor is it entitled to know, the
reason for the temporary failure status code being returned;
greylisting could be in effect, or it could be caused by a local
resource issue at the server. A client therefore needs to be
equipped to retry in order to be considered fully capable.)
The counterargument to this "false positive" problem is that email
has always been a "best-effort" mechanism; thus, this cost is
ultimately low in comparison to the cost of dealing with high volumes
of unwanted mail. Still, the actual effect of such delays can be
significant, such as altering the tone or flow of a multi-participant
discussion to a mailing list.
When the clients are subjected to any kind of reconfiguration,
especially network renumbering, the cache of information stored about
SMTP client history does not benefit legitimate clients that are
already listed for acceptance. To the greylisting implementation,
such clients are once again unknown, and they will once again be
subjected to the delay.
Another obvious cost is for the required database. It has to be
large enough to keep the necessary history and fast enough to avoid
excessive inefficiencies in the server's operations. The primary
consideration is the maximum age of records in the database. If
records age out too soon, then hosts that do retry per [SMTP] will be
periodically subjected to greylisting even though they are well-
behaved; if records age out after too long a period, then eventually
spamware that launches a new campaign will not be identified as
"unknown" in this manner and will not be required to retry.
Presuming that known friendly senders will be manually configured as
exceptions to the greylisting check, a steady state will eventually
be reached wherein the only mail that is delayed is mail from an IP
address that has never sent mail before. Experience suggests that
the vast majority of mail comes from places on a developed exception
list, so after a training period, only a small proportion of mail is
actually affected. The training period could be replaced by
processing a history of email traffic and adding the IP addresses
from which most traffic arrives to the exception list.
Applying greylisting based on actual message content (i.e., post-
DATA) is substantially more expensive than any of the other
alternatives both in terms of the resources required to accept and
temporarily store a complete message body (which can be quite
substantial) and any processing that is done on that content. As a
consequence, such methods incur more cost during the session and thus
are not typical practice.
4. Unintended Consequences
4.1. Unintended Mail Delivery Failures
There are a few failure modes of greylisting that are worth
considering. For example, consider an email message intended for
firstname.lastname@example.org. The example.com domain is served by two receiving
mail servers, one called mail1.example.com and one called
mail2.example.com. On the first delivery attempt, mail1.example.com
greylists the client, and thus the client places the message in its
outgoing queue for later retry. Later, when a retry is attempted,
mail2.example.com is selected for the delivery, either because
mail1.example.com is unavailable or because a round-robin [DNS]
evaluation produces that result. However, the two example.com hosts
do not share greylisting databases, so the second host again denies
the attempt. Thus, although example.com has sought to improve its
email throughput by having two servers, it has, in fact, amplified
the problem of legitimate mail delay introduced by greylisting.
Similarly, consider a site with multiple outbound MTAs that share a
common queue. On a first outbound delivery attempt to example.com,
the attempt is greylisted. On a later retry, a different outbound
MTA is selected, which means example.com sees a different source, and
once again greylisting occurs on the same message. The same effect
can result from the use of [DHCP], where the IP address of an
outbound MTA changes between attempts.
For systems that do DATA-level greylisting, if any part of the
message has changed since the first attempt, the tuple constructed
might be different than the one for the first attempt, and the
delivery is again greylisted. Some MTAs do reformulate portions of
the message at submission time, and this can produce visible
differences for each attempt.
A host that sends mail to a particular destination infrequently might
not remain "known" in the receiving server's database and will
therefore be greylisted for a high percentage of mail despite
possibly being a legitimate sender.
All of these and other similar cases can cause greylisting to be
applied improperly to legitimate MTAs multiple times, leading to long
delays in delivery or ultimately the return of the message to its
sender. Other side effects include out-of-order delivery of related
Address translation technologies such as [NAT] cause distinct MTAs to
appear to come from a common IP address. This can cause greylisting
to be applied only to the first connection attempt from the shared IP
address, meaning future MTAs connecting for the first time will be
exempted from the protection greylisting provides.
4.2. Unintended SMTP Client Failures
Atypical SMTP client behaviors also need to be considered when
Some clients do not retry messages for very long periods. Popular
open source MTAs implement increasing backoff times when messages
receive temporary failure messages and/or degrade queue priority for
very large messages. This means greylisting introduces even more
delay for MTAs implementing such schemes, and the delay can become
large enough to become a nuisance to users.
Some clients do not retry messages at all, in violation of [SMTP].
This means greylisting will cause outright delivery failure right
away for sources, envelopes, or messages that it has not seen before,
regardless of the client attempting the delivery, essentially
treating legitimate mail and spam the same.
If a greylisting scheme requires a database record to have reached a
certain age rather than merely testing for the presence of the record
in the database, and the client has a retry schedule that is too
aggressive, the client could be subjected to rate limiting by the MTA
independent of the restrictions imposed by greylisting.
Some SMTP implementations make the error of treating all error codes
as fatal, contrary to [SMTP]; that is, a 4yz response is treated as
if it were a 5yz response, and the message is returned to the sender
as undeliverable. This can result in such things as inadvertent
removal from mailing lists in response to the perceived rejections.
Some clients encode message-specific details in the address parameter
to the [SMTP] MAIL command. If doing so causes the parameter to
change between retry attempts, a greylisting implementation could see
it as a new delivery rather than a retry and disallow the delivery.
In such cases, the mail will never be delivered and will be returned
to the sender after the retry timeout expires.
A client subjected to greylisting might move to the next host found
in the ordered [DNS] MX record set for the destination domain and re-
attempt delivery. This has several considerations of its own:
o Traffic to those alternate servers increases merely as a result of
o Alternate (MX) servers SHOULD share the same greylisting database.
When they do not -- as is often true when the servers occupy
different Administrative Management Domains (ADMDs) -- SMTP
clients can see variable treatment if they try to send to
different MX hosts.
o When alternate MX servers relay mail back to the "primary" MX
server, the latter SHOULD be configured to permit the other
servers to relay mail without being subjected to greylisting.
There are some applications that connect to an SMTP server and
simulate a transaction up to the point of sending the RCPT command in
an attempt to confirm that an address is valid. Some of these are
legitimate applications (e.g., mailing list servers), and others are
automated programs that attempt to ascertain valid addresses to which
to send spam (a "directory harvesting" attack). Greylisting can
interfere with both instances, with harmful effects on the former.
4.3. Address Space Saturation
Greylisting is obviously not a foolproof solution to avoiding abusive
traffic. Bad actors that send mail with just enough frequency to
avoid having their records expire will never be caught by this
mechanism after the first instance.
Where this is a concern, combining greylisting with some form of
reputation service that estimates the likely behavior for IP
addresses that are not intercepted by the greylisting function would
be a good choice.
The following practices are RECOMMENDED based on collected
1. Implement greylisting based on a tuple consisting of (IP address,
RFC5321.MailFrom, and the first RFC5321.RcptTo). It is
sufficient to use only the first RFC5321.RcptTo as legitimate
MTAs appear not to reorder recipients between retries. Including
RFC5321.MailFrom improves accuracy where the IP address is being
matched in clusters (e.g., CIDR blocks) rather than precisely
(see below). After a successful retry, allow all further [SMTP]
traffic from the IP address in that tuple regardless of envelope
2. Include a configurable range of time within which a retry from a
greylisted host is considered and outside of which it is
otherwise ignored. The range needs to cover typical retry times
of common MTA configurations, thus anticipating that a fully
capable MTA will retry sometime after the beginning of the range
and before the end of it. The default range SHOULD be from one
minute to 24 hours. Retries within the range are permitted and
satisfy the greylisting test, and the client is thus no longer
likely to be a sender of spam. Retries after the end of the
range SHOULD be considered to be a new message for the purposes
of greylisting evaluation (i.e., reset the "first seen" timestamp
for that IP address). Some sites use a higher time value for the
low end of the time range to match common legitimate MTA retry
timeouts, but additional benefit from doing so appears unlikely.
3. Include a timeout for database entries, after which records for
IP addresses that have generated no recent traffic are deleted.
This step is intended to re-enable greylisting for an IP address
in the event that it has changed "owners" and will subject the
client to another round of greylisting. The default SHOULD be at
least one week.
4. For an Administrative Management Domain (ADMD), all inbound
border MTAs listed in the [DNS] SHOULD share a common greylisting
database and common greylisting policies. This handles sequences
in which a client's retry goes to a different server after the
first 4yz reply, and it lets all servers share the list of hosts
that did retry successfully.
5. To accommodate those senders that have clusters of outgoing mail
servers, greylisting servers MAY track CIDR blocks of a size of
its own choosing, such as /24, rather than the full IPv4 address.
(Note, however, that this heuristic will not work for clusters
having machines on different networks.) A similar grouping
capability MAY be established based on the domain name of the
mail server if one can be determined.
6. Include a manual override capability for adding specific IP
addresses or network blocks that always bypass checks. There are
legitimate senders that simply don't respond well to greylisting
for a variety of reasons, most of which do not conflict with
[SMTP]. There are also some highly visible online entities such
as email service providers that will be certain to retry; thus,
those that are known SHOULD be allowed to bypass the filter.
7. Greylisting SHOULD NOT be applied by an ADMD's submission service
(see [SUBMISSION]) for authenticated client hosts. It also
SHOULD not be applied against any authenticated ADMD session.
Authentication can include whatever mechanisms are deemed
appropriate for the ADMD, such as known internal IP addresses,
protocol-level client authentication, or the like.
There is no specific recommendation as to the specific choice of 4yz
code to be returned as a result of a greylisting delay. Per [SMTP],
however, the only two reasonable choices are 421 if the
implementation wishes to terminate the connection immediately and 450
otherwise. It is possible that some clients treat different 4yz
codes differently, but no data is available on whether using 421
versus some other 4yz code is particularly advantageous.
There is also no specific recommendation as to the choice of text to
include in the SMTP reply, if any. Some implementers argue that
indicating that greylisting is in effect can give spamware a hint as
to when to try again for successful delivery, while others suspect
that it won't matter to spamware and thus the more likely audience is
legitimate senders seeking to understand why their mail is being
6. Measuring Effectiveness
A few techniques are common when measuring the effectiveness of
greylisting in a particular installation:
o Arrange to log the spam versus legitimate determinations of
messages and what the greylisting decision would have been if
enabled; then determine whether there is a correlation (and, of
course, whether too much legitimate email would also be affected).
o Continuing from the previous point, query the set of IP addresses
subjected to greylisting in any popular [DNSBL] to see if there is
a strong correlation.
7. IPv6 Applicability
The descriptions and recommendations presented in this memo are based
on many years of experience with greylisting in the IPv4 Internet
environment, so they clearly pertain to IPv4 deployments only.
The greater size of an IPv6 address seems likely to permit
differences in behaviors by bad actors, and this could well mean
needing to alter the details for applying greylisting; it might even
negate any benefits in using greylisting at all. At a minimum, it is
likely to call for different specific choices for any greylisting
In addition, an obvious consideration is that the size of the
database required to store records of all of the IP addresses seen
will likely be substantially larger in the IPv6 environment.
8. Security Considerations
This section discusses potential security issues related to
The discussion above highlights the fact that, although greylisting
provides some obvious and valuable defenses, it can introduce
unintentional and detrimental consequences for delivery of legitimate
mail. Where timely delivery of email is essential, especially for
financial, transactional, or security-related applications, the
possible consequences of such systems need to be carefully
Specific sources can be exempted from greylisting, but, of course,
that means they have elevated privilege in terms of access to the
mailboxes on the greylisting system, and malefactors can seek to
The database that has to be maintained as part of any greylisting
system will grow as the diversity of its SMTP clients' hosts grows
and, of course, is larger in general depending on the nature of the
tuple stored about each delivery attempt. Even with a record aging
policy in place, such a database could grow large enough to interfere
with the system hosting it, or at least to a point at which
greylisting service is degraded. Moreover, an attacker knowing which
greylisting scheme is in use could rotate parameters of SMTP clients
under its control, in an attempt to inflate the database to the point
Implementers could consider configuring an appropriate failure policy
so that something locally acceptable happens when the database is
attacked or otherwise unavailable.
In practice, this has not appeared as a serious concern, because any
reasonable aging policy successfully moderates database growth. It
is nevertheless identified here as a consideration as there may be
implementations in some environments where this is indeed an issue.
9.1. Normative References
[EMAIL-ARCH] Crocker, D., "Internet Mail Architecture", RFC 5598,
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
[SUBMISSION] Gellens, R. and J. Klensin, "Message Submission for
Mail", STD 72, RFC 6409, November 2011.
9.2. Informative References
[CIDR] Fuller, V. and T. Li, "Classless Inter-domain Routing
(CIDR): The Internet Address Assignment and Aggregation
Plan", BCP 122, RFC 4632, August 2006.
[DHCP] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, March 1997.
[DNS] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[DNSBL] Levine, J., "DNS Blacklists and Whitelists", RFC 5782,
[MAIL] Resnick, P., Ed., "Internet Message Format", RFC 5322,
[NAT] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022,
[PUREMAGIC] Harris, E., "The Next Step in the Spam Control War:
Greylisting", August 2003,
[SAUCE] Jackson, I., "GNU SAUCE", 2001,
Appendix A. Acknowledgments
The authors wish to acknowledge Mike Adkins, Steve Atkins, Mihai
Costea, Derek Diget, Peter J. Holzer, John Levine, Chris Lewis, Jose-
Marcio Martins da Cruz, John Klensin, S. Moonesamy, Suresh
Ramasubramanian, Mark Risher, Jordan Rosenwald, Gregory Shapiro, Joe
Sniderman, Roland Turner, and Michael Wise for their contributions to
this memo. The various participants of the MAAWG Open Sessions about
greylisting were also valued contributors.
Murray S. Kucherawy
128 King St., 2nd Floor
San Francisco, CA 94107
Phone: +1 415 946 3800
675 Spruce Dr.
Sunnyvale, CA 94086