There are a number of dedicated spam operations flooding the net with unwanted junk mail from known, fixed locations. Many network administrators have started to filter out all connections from these so-called spamhaus operations. The spammers have had to develop new techniques to evade the blockades. Their current favorite is to hijack a third-party mail server. The spammers, in effect, launder their junk email through third-party relays to slip through the spam filters.
Spammers use relays to increase the number of messages they can spew.
A lowly PC sitting at the end of a phone line can only pump out a limited number of messages. Further, if the spammer can relay through several mail servers in parallel, they can flood the net with extraordinary amounts of junk mail. Spammers can hide behind third-party relays. If a spammer sends junk email directly, network managers can trace back the connection and deal with the problem. If, instead, the spammer relays the mail, they may be able to obscure their identity.