sendmail milters

I finally took the plunge today and got milter-ahead working. I had been meaning to do something about it for ages, but didn’t have a spare redhat machine to test it on.
What does it do?
The concept behind the milter is very simple.
At the start of the SMTP transaction the milter connects with the destination mail server to see if the recipient is valid. If the recipient is valid or the mail server accepts mail for it (which is not always the same thing) then the SMTP transaction continues. If, on the other hand, the destination mail server rejects the recipient then the transaction stops there.
It also caches the results, so you don’t have to make a connection for each and every mail.
Why is this useful?
If you are using a gateway for multiple mail servers the milter reduces load and bandwidth on both the gateway and the receiving mail server.
Some “older” domains can get hit with dictionary attacks several times a day, so the milter can reduce the load on your MailScanner processes significantly.