SMTP is defined in RFC 821, and is supplemented by several other RFCs that define a variety of extensions to the core protocol. For example, RFC 821 states that a reliable protocol (such as TCP) must be used to transfer mail messages between systems, although it also states that any reliable transport protocol can be used (such as X.25, or even SPX). When TCP is used for SMTP - as it is on the Internet - then servers are required to listen on TCP port 25. This port is now reserved as a well-known service port exclusively for SMTP's use.
All of this is defined in RFC 821, the documented standard for the SMTP mail protocol.
It is important to note that SMTP does not deliver mail to users' mailboxes directly. Instead, it is only used to send and receive mail between message stores. The message store must use a delivery agent to actually place mail into the appropriate recipient's message store. The delivery agent may be a Microsoft Mail or cc:Mail gateway for SMTP, or may be an external application such as Unoverica Message Transport's UCMSEND.NLM.
For more information about the SMTP protocol, refer to section D.2.1 SMTP's Command/ Response Syntax. For more information about SMTP's restrictions, refer to section D.2.3 What SMTP Doesn't Do. For more information about TCP, refer to section B.5 Transport-Layer Protocols and Services.
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