This is caused by misconfigured mail servers on the sending end which have found themselves on the blacklist. Most mail-transfer agents let you specify exceptions to the blacklists if there are a few machines whose transmissions you want to let through. Some spammers aren't yet in the blacklist. Using sendmail's 'access table' feature, I have my own personal blacklist of several dozen spam originators. In the past, I have even blacklisted entire domains.
Mailing lists One last source of e-mail annoyance is mailing lists.
I subscribe to several mailing lists and, on some of them, useless threads recur leading to flame wars and general irritation. However, you still get all the flame-filled followups. For that reason, I wrote a small perl script called ThreadKiller , which kills entire threads on a mailing list. ThreadKiller then examines each message and, if it refers to a killed message, the message itself is killed and added to the database of dead messages. You run a cron job once a night which cleans the database of messages older than a certain number of days.