Network administrators have used it to debug mail connectivity. It has been used to route around known mail problems. Although it is rarely needed, it has proven useful on occasions. These days, however, the legitimate uses of mail relay are dwarfed by the number of mailer hijackings. A hijacking occurs when massive amounts of mail are relayed through a server.
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. If, however, the spammer can grab a hold of a high-powered mail host with a super fast ethernet connection, they can push through hundreds of times more 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.