milter-greylisting

Some months ago, a friend was talking about greylisting and how effective it was against spam and e-mail viruses. Today I installed milter-greylisting on two of my mailservers.
And it is quite amasing. The first time a SMTP server tries to deliver a message, the maessage is rejected, real mailservers will resend the message and this time the message is accepted. A mass mailing application or a virus will most likely not handle the error response correctly, and just ignore it.
This will cause a delay in the e-mail delivery depending on the configuration on your server, but then again the ammount of spam will be next to nothing.