Simple Mail Transfer Protocol (SMTP) is an Internet standard for electronic mail (e-mail) transmission across Internet Protocol (IP) networks. SMTP was first defined in RFC 821 (STD 15), and last updated by RFC 5321 (2008) which includes the extended SMTP (ESMTP) additions, and is the protocol in widespread use today.

Enforcing the requirements of the Simple Mail Transfer Protocol (SMTP) can be useful in blocking email from computers that do not comply with the RFC standards. Many spammers use poorly written software or are unable to abide by the standards because they do not have legitimate control of the computer sending spam (Zombie computers). By applying certain restrictions to the MTA a mail administrator can reduce the spam that passes through significantly.

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