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, 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 a single address or a range). If a match is found, the connection is closed with a response (455) refusing reception.

If it passes the peer-address check, the proxy thread next attempts the typical proxy rule match steps ("Policy Rule Matching"), except that only the source address is checked. For each rule that matches, an SMTP connection is attempted to the message transfer agents(MTA) listed as destination for the rule.

Once a connection to a destination server MTA has been established, data are relayed between the client and server MTAs. The content is scanned for commands that introduce source mailbox, destination mailboxes, and the data stream itself. Source mailboxes are checked against the spam list (if any). The destination mailboxes are checked against the relay list.

If configured for content scanning, the body of the e-mail messages which pass the above-mentioned spam and access control mechanisms, are fed to the scanner for inspection. The scanner may instruct that the content be blocked, or may alter the content (clean viruses from it, for example) , or may return it unaltered. Scanning which results in content alteration is reflected in the e-mail messages so modified.. Scanning results are recorded in the SunScreen log entries regarding SMTP transfers.