Dirk große Osterhues

*[MTA]: Mail Transfer Agent
In this article the approach of greylisting as a protection against spam in Exim 4 is being described. The concepts of greylisting temporarily rejects mails at SMTP time. A regular MTA without spamming intent will try to send mail again after a certain period of time. The average trojan or worm on the other hand usually sends spam in waves. It will try to send its load only once and won't try again later. This concept facilitates spam reduction to a huge extent.

Setting up greylisting in Ubuntu Linux using Exim 4 is a breeze. To work, it simply needs the following lines.

Firstly, the installation of the package greylistd:

aptitude install greylistd

To activate the new package in Exim's configuration, the following command will do some magic:

greylistd-setup-exim4 add

The default configuration of greylistd is a bit too paranoid from my point of view. Therefore I adjust /etc/greylistd/config as follows (usually the last two existing entries in that file):

singlecheck = true
singleupdate = true

This has the effect that a domain that has already been verified won't become greylisted again if a different user sends from that particular domain. Also, it enables for manual manipulation of the whitelist by just adding IPs of "good" MTAs.

The whitelist file that is supposed to be edited by admins resides at /etc/greylistd/whitelist-hosts. The predifined whitelist can be found at /var/lib/greylistd/whitelist-hosts.