Greylisting
The LinuxRulz mailserver network employs a form of fighting spam called greylisting. In
name as well as working, greylisting is related to whitelisting and blacklisting. What
happens is that each time a given mailbox receives an email from an unknown contact (ip),
that mail is rejected with a "try again later" message (This happens at the SMTP layer
and is transparent to the end user). This, in the short run, means that all mail gets
delayed atleast until the sender tries again, this is where spam loses out! Most spam is
not sent out using RFC compliant MTAs (Mail Transfer Agent), the spamming software will
not try again later.
If the MTA tries again after the greylisting period (4 minutes), the triplet (sender email
address, senders server IP and recipient email address) is whitelisted for 7 days. If email
from the same sender to the same recipient is received within 7 days it will not be delayed
and will reset the whitelisting period for a further 7 days.
Various analysis tools are run to generate and use statistics from greylisting and may
automatically blacklist some hosts exhibiting bad or abusive behavior.
Errors receiving mail?
If you are having trouble receiving mail and the error returned to the sender contains
"Recipient address rejected: Policy Rejection: Greylisting - Please try later.", it means
the senders mailserver must try again 4 minutes after its first attempt. Seeing as you're
not receiving the email and the sender is getting an error it means that there is something
wrong, the senders email server is not trying again. Please have the sender contact his ISP
or email service provide to find out what the problem is.
The folowing RFC's describe in detail the retrying of temporarily rejected delivery
attempts.
RFC 2821, page 42/3, section 4.2.1 describes error 45x as being temporary mail error.
RFC 2821, page 58, section 4.5.4.1 states the following "The sender MUST delay retrying a particular destination after one attempt has failed. In general, the retry interval SHOULD be at least 30 minutes;....".
References
More information on Greylisting
SMTP Standard - RFC2821