TEG Design - Technologic e-Generation - Reference : SMTP Commands:

This page gives a quick summary of the major commands and command sequence used by the SMTP Protocol to transfer email. In Unix, the “sendmail” program listens for connections on port 25, and the SMTP protocol is a plain text conversation. This means that it is possible to telnet to port 25 on a system and manually enter SMTP commands. This is useful for two things, manually testing a sendmail configuration, and forging email.

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Tim Fehlman