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RFC 821 and security

SMTP was first implemented when Internet was a network much different than the one we use today. Relaying back then was not frowned upon, as it was a legitimate and sometimes necessary way of sending messages to or from hosts that were not constantly connected. RFC 821 provides even specific commands for relaying, in the form


This is the first point we should note -- This syntax should not be allowed by the wrapper. Current Sendmail versions usually rejects this syntax, but it should not be overlooked.

Gunnar Wolf