Mac® Security Bible

28.4. Configuring Relay Options

SMTP servers are usually described as being for outgoing mail, but that's an oversimplification. It's true that when your email client wants to send a message to someone else, it connects to an SMTP server. But what happens next is that your SMTP server connects to another SMTP server (either the one the recipient uses or an intermediate server that sends the message one step closer to it) in order to deliver the message to its destination. Conversely, when someone sends you a message, the other person's SMTP server talks to your SMTP server to exchange the data. So, in fact, SMTP servers both send and receive email. And then, once a message is on its destination server, POP or IMAP software takes over to handle delivering the message to the user's email client.

Whenever an SMTP server is being used to send a message to another server — whether that message originated from a local user, a remote email client, or an upstream SMTP server —it's said to be a relay. That means about half of what any SMTP server does is relaying; the other half is receiving email destined for a local user. Mac OS X Server's SMTP server can deliver messages directly to their destination server (the usual case) or it can be configured to relay all outgoing messages to some other SMTP server.

Relaying, then, is a proper and necessary part of any SMTP server's operation. However, the one thing you probably don't want is for just anyone to be able to send (relay) mail through ...