Spam is the black plague of the Internet world and this Sendmail milter
attempts to address a small subset of the problem by verifying that the 
sender's email address is in good standing by performing an SMTP callback 
to the MX server responsible for the sender's domain. This milter is not 
intended to be a complete solution to spam and so it should be used in 
conjunction with other tools such as DNS blacklists and content filters.

To be in good standing, the MX server of the sender must be reachable and
willing to accept email for the sender from the Delivery Status 
Notification (DSN) address, which is the null address <> used for error 
reporting. This particular requirement is similar to using the 
RFC-Ignornant DSN Blacklist.

During the callback, any failure in establishing an SMTP connection such 
as no DNS entry or the primary MX server is unreachable, or during the 
SMTP dialogue such as not accepting the DSN, user unknown, or mail box is 
full, etc. will result in the incoming mail in question being rejected. 
In the case of a temporary error during the SMTP dialogue with the 
sender's MX server, milter-sender will reject the current message with 
a temporary error code, which the sender is then free to retry again 
later. DNS lookup and connection problems are treated as temporary errors.