SMTP is designed to provide a simple transport for moving mail between dissimilar hosts and mail systems. It does not deliver mail directly to your desktop PC (unless you happen to have an SMTP server running there). Most networks use a central, shared postoffice that is running on a fileserver or multi-user host. Since SMTP requires that the SMTP server is always running, the resource requirements (disk, memory, CPU time, etc.) are quite high. Therefore, running an SMTP server on a desktop PC is unlikely, since they aren't up all the time, and do not typically have the necessary resources.
Nor does SMTP deliver mail to users' mailboxes directly. Instead, it relies on a delivery agent to perform this work. 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.
Finally, SMTP does not define the content or the formatting of the message. These services are documented in RFC 822. It is up to the mail system in use (whether cc:Mail, SENDMAIL, or whatever) to translate the RFC 822 headers and body parts into a format that is usable by the local mail system.
For more information on the structure of SMTP mail messages, refer to Appendix E: Message Envelope, Header, Body and Parts.
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