A Mail Relay is a server, normally on the Internet, which can forward mail from its users to other mail servers. Most electronic mail, and most Mail Relay's, use the SMTP mail transfer protocol.
Since most mail client programs (e.g., Netscape, Eudora, Outlook) do not fully understand how to deliver mail, they generally rely on a Mail Relay to take care of mail delivery. Consequently, Mail Relay's are mandatory for most mail systems.
A potential security problem with Mail Relay is that, if they are not correctly configured, they will agree to relay mail sent by any user, from any network. This allows evil Spam-mers to forward mail through them, and thus make their Spam appear to have originated from the offending Mail Relay.