SMTP Proxy Operation

When the SMTP proxy starts, it reads its policy files, determines its local server name for use in relay checking, and listens on the standard SMTP port (25) for connections. When a connection is made, the SMTP proxy starts a new thread to handle the connection, and the main thread resumes listening.

The child thread takes control of the connection from the client. It then attempts to reverse-translate the address of the client (from the connection state) to yield a registered name.

If a registered name is discovered, then the suffixes in the mail_spam list are checked against that name. If a suffix matches (the end of) the name of the originating host, the connection is closed with a response (455) refusing reception.

If no name is registered for the address, then the address itself is sought in the mail_spam list (looking for items that contain single or range IP addresses). If a match is found, the connection is closed with a response (455) refusing reception.

Having passed the peer-address check, the proxy thread next attempts the typical proxy rule match steps (given earlier), except that only the source address is checked. For each rule that matches, an SMTP connection is attempted to the MTAs listed as destination for the rule.