Greylisting (or graylisting) is a method of defending e-mail users against spam. A mail transfer agent (MTA) using greylisting will "temporarily reject" any email from a sender it does not recognize. If the mail is legitimate the originating server will, after a delay, try again and, if sufficient time has elapsed, the email will be accepted.
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Typically, a server employing greylisting will record the three pieces of data known as a "triplet" for each incoming mail message:
This is checked against the mail server's internal database. If this triplet has not been seen before (within some configurable period), the email is greylisted for a short time (also configurable), and it is refused with a temporary rejection with a SMTP 4xx error code. The assumption is that since temporary failures are defined in the SMTP-related RFCs, a legitimate server will try again to deliver the email.
The temporary rejection can be issued at different stages of the SMTP dialogue, allowing an implementation to store more or less data about the incoming message. The trade-off is more work and bandwidth for more exact matching of retries with original messages.[1] Rejecting a message after its content has been received allows the server to store a choice of headers and/or a hash of the message body.
Large companies with big pools of sending machines typically break RFCs and greylisting systems by returning greylisted mail to the sending pool of servers, but to be fully compliant the sending server and its corresponding unique IP address are solely responsible for the delivery of the message. The practice of returning a message back into a pool for later delivery breaks this rule and is generally discouraged and will cause mail delays. Greylisting can generally be overridden by a fully validated TLS connection with a matching certificate. Because large senders often have a pool of machines that can send (and resend) email, IP addresses that have the most-significant 24 bits (/24) the same are treated as equivalent, or in some cases SPF records are used to determine the sending pool. Similarly, some e-mail systems use unique per-message return-paths, for example variable envelope return path (VERP) for mailing lists, Sender Rewriting Scheme for forwarded e-mail, Bounce Address Tag Validation for backscatter protection, etc. If an exact match on the sender address is required, every e-mail from such systems will be delayed. Some greylisting systems try to avoid this delay by eliminating the variable parts of the VERP by using only the sender domain and the beginning of the local-part of the sender address.
Greylisting is effective because many mass email tools used by spammers do not queue and reattempt mail delivery as is normal for a regular Mail Transport Agent. They do not queue and resend email as this requires the expenditure of resources (spamming normally operates on very narrow margins). Since the advent of greylisting spammers have taken to rerunning their mail delivery tools to resend the same email again without having to expend resources queueing the email. This approach still requires them to expend additional resources resending the email the second time though. As of 2011 spammers have been using this technique for many years.
Delaying delivery also gives real-time blackhole lists and similar lists time to identify and flag the spam source. Thus, these subsequent attempts are more likely to be detected as spam by other mechanisms than they were before the greylisting delay.
The main advantage from the users' point of view is that greylisting requires no additional configuration from their end. If the server utilizing greylisting is configured appropriately, the end user will only notice a delay on the first message from a given sender, so long as the sending email server is identified as belonging to the same whitelisted group as earlier messages. If mail from the same sender is repeatedly greylisted it may be worth contacting the mail system administrator with detailed headers of delayed mail.
From a mail administrator's point of view the benefit is twofold. Greylisting takes minimal configuration to get up and running with occasional modifications of any local whitelists. The second benefit is that rejecting email with a temporary 451 error (actual error code is implementation dependent) is very cheap in system resources. Most spam filtering tools are very intensive users of CPU and memory. By stopping spam before it hits filtering processes, far fewer system resources are used. This allows more layers of spam filtering or higher throughput since greylisting can easily be configured as a first line of defense with a heuristic filter such as SpamAssassin handling messages that go through.
Some greylisting packages support a SQL backend which allows for a distributed multiple-server frontend to be deployed with the same greylisting data on all frontends.
The biggest disadvantage of greylisting is that it destroys the near-instantaneous nature of email that users have come to expect. Mail from unrecognised senders is delayed by typically about 15 minutes, and up to four hours. A customer of a greylisting ISP can not always rely on getting every email in a pre-determined amount of time.
Modern greylisting applications (such as Postgrey) attempt to evaluate the reputation of an incoming email based on information in its database. For example, email from an IP address that has been a source of non-spam in the past may be whitelisted even if that particular sender has not been seen before. By using techniques like this the advantages of greylisting can be retained while delays in legitimate email can be reduced.
The original specification for email states that it is not a guaranteed delivery mechanism and not an instantaneous delivery mechanism. This means that greylisting is a perfectly legitimate process and does not break any protocols or rules.
If mail from a particular frequent sender is sent from any of several mail servers, mail may be delayed unless the greylisting server recognises the different servers as belonging to the same whitelisted group.
On a technical level, some SMTP clients and SMTP servers acting as clients may interpret the temporary rejection as a permanent failure. Old clients conforming only to the obsolete specification (RFC 821) and ignoring its recommendations may give up on delivery after the first failed attempt—RFC 821 states that clients "should" retry messages rather than using the word "must". RFC 2119 dictates that "should" means recommended and to ignore at your own risk, and it is a violation of the current SMTP standard for the client to fail to retry. The current SMTP specification (RFC 5321) clearly states that "the SMTP client retains responsibility for delivery of that message" (section 4.2.5) and "mail that cannot be transmitted immediately MUST be queued and periodically retried by the sender." (section 4.5.4.1).
This problem can affect SMTP clients in unexpected ways. Most MTAs will queue and retry messages, but a small number do not.[2] A similar concern exists for applications which act as SMTP clients and fail to incorporate any form of queueing for deferred SMTP mail. This can be mitigated on the sending side by configuring the application to use a local SMTP server as an outbound queue, instead of attempting direct delivery. For the server operator who uses greylisting, clients which are known to fail on temporary errors can be supported by whitelisting or exception lists.
Some MTAs, upon encountering the temporary failure message from a greylisting server on the first attempt, will send a warning message back to the original sender of the message.[2] The warning message is not a bounce message, but it is often formatted similarly to one and reads like one. This practice often causes the sender to believe that the message has not been delivered, when in fact the message will be delivered successfully at a later time.
When a mail server is greylisted, the duration of time between the initial delay and the re-transmission is variable. Some mail servers use a default of four hours, though most will retry sooner. Most open-source MTAs have retry rules set to attempt delivery after around fifteen minutes (Sendmail default is 0, 15, ..., Exim default is 0, 15, ..., Postfix default is 0, 16.6, ..., Qmail default is 0, 6:40, 26:40, ..., Courier default is 0, 5, 10, 15, 30, 35, 40, 70, 75, 80,...). Microsoft Exchange defaults to 0, 1, 2, 22, 42, 62 ..., Message Systems Momentum defaults to 0, 20, 60, 100, 180, ...
Greylisting delays much of the mail from non-whitelisted mail servers—not just spam—until typical patterns of communication are recorded by the greylisting system. For best results, whitelisting should be used extensively. A static list of public servers worth being whitelisted can be found in the greylisting.org repository.
Also, legitimate mail might not get delivered if the retry does not arrive within the time window the greylisting software uses, or if the retry comes from a different IP address from the original attempt. When the source of an email is a server farm or goes out through an anti-spam mail relay service, it is likely that on the retry a server other than the original server will make the next attempt. Since the IP addresses will be different, the recipient's server will fail to recognize that the two attempts are related and refuse the latest connection as well. This can continue until the message ages out of the queue if the number of servers is large enough. Such server farming techniques can be construed as breaking RFCs detailed above since the original sending machine has absolved itself of the responsibility of mail delivery by tossing it back into the pool, which breaks the state of the mail delivery process. This problem can partially be bypassed by identifying and whitelisting such server farms in advance. However, it is not possible on a distributed network the size of the Internet to maintain a complete list of all such server farms.[3]
Greylisting can be a particular nuisance with websites that require an account to be created and the email address confirmed before they can be used. If the sending MTA of the site is poorly configured, greylisting may delay the initial email containing the signup confirmation link, thus introducing a waiting period even though the actual website may have attempted to send out the email confirmation code immediately. Almost all stock-configured Sendmail MTAs (sendmail being the most widely deployed MTA on the internet) will retry after a few minutes, leading to typical delays of under 10 minutes in most cases (still dependent on the greylisting configuration).
Greylisting is particularly effective in many cases at weeding out misconfigured MTAs, and is gaining in popularity as a very effective anti-spam tool. It is likely that those MTAs that do not correctly handle greylisting will become less numerous as greylisting spreads.
In order for greylisting to work for a particular domain, all backup mail servers (as specified by lower-priority MX records for the domain) must implement the greylisting policy as well.
Also, if certain details of the sending vary and the receiving MTA is not programmed to notice this, a message may be greylisted eternally and never delivered.
Greylisting will cause longer delivery delays if the sender has a large infrastructure and is sending from a different IP when it retries. However this technically breaks SMTP protocol rules, since delivery is the responsibility of the sending server and its associated IP address, and "tossing it back into a pool" for retry by a different server in the group breaks this continuity, and will quite correctly and legitimately restart the greylisting process over again, since delivery is being retried from a different server. Technically there is no reason to throw a message back into a pool for retry by a different server, if load balancing and capacity handling require multiple outbound servers then performing the load balancing prior to insertion into the queue of a delivery server is a simple and obvious way round this which allows for the SMTP protocol to be adhered to and avoids the legitimate re-greylisting by any receiving MTA when the sender IP address changes. In this case it is clearly the sending systems infrastructure implementation that indrectly causes the delay by virtue of its inability to maintain delivery retries from a single IP address.