GREYLIST(7)

NAME
greylist - how the postoffice greylist works

DESCRIPTION
A greylist is a way to slow down spammers by temporarily refusing access to the mail server. The greylist used by the postoffice(8) SMTP server is a @NDBM@(3) database containing the MAIL FROM: and IP addresses of SMTP clients, the time they are permitted to send mail, and the time they last sent mail.

When a client connects to the SMTP server and issues a MAIL FROM: command, the server looks up that username and the IP address that the client is calling from in the greylist. If it's later than the time the client is permitted to send mail, RCPT TO: commands will be processed normally, otherwise the mail server will refuse the command with a temporary failure error.

In postoffice(8), new connections are typically greylisted for 60 minutes with longer delays for connections coming from

a   Hosts that do not have a valid reverse DNS entry (twice the regular delay), or

b   mail from the MAILER-DAEMON at the client (four times the regular delay; five times if the host doesn't have a valid reverse DNS entry.)

Furthermore, mail from the MAILER-DAEMON is only accepted for 10 minutes after the greylist entry expires. This may be a bug, or it may not. On the machine Pell, there are a grand total of two real users and on any given day there are up to 500 pieces of mail coming from MAILER-DAEMON. And on a typical day, zero of those pieces of mail are real messages. Junk mail can cool its heels for a long long time as far as I'm concerned.

BUGS
If mail is sent from a domain that uses a lot of machines to round-robin outgoing mail, it will be delayed for a much longer time than the nominal 60 minute delay. If the mail is from the MAILER-DAEMON at that host, it may never be accepted by the machine with the greylist.

AUTHOR
David Parsons

SEE ALSO
@NDBM@(1), smtpauth(5), postoffice(8), authexpire(8);

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