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I want to implement my own SMTP server for learning and personal interest purposes, and have some concerns/questions about SMTP protocol.

The SMTP protocol has it's initial RFC-821, but has been updated/extended several times over the years. As SMTP's wiki page says:

The ESMTP format was restated in RFC 2821 (superseding RFC 821) and updated to the latest definition in RFC 5321 in 2008

But when I check RFC 5321, I still see Updated by: 7504. Why is that?

Is there one RFC about SMTP that covers it fully up to current date? And if not, what's the reading path/order I should follow to get familiar with SMTP and make sure I'm not missing anything?

Cxovrika
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    As you noticed yourself, RFCs are not an ideal place to learn about SMTP (or any other protocol). Have a look at https://explained-from-first-principles.com/email/#delivery-protocols to learn about (E)SMTP and its extensions. (Disclaimer: I wrote this myself.) – Kaspar Etter May 22 '21 at 16:07

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Start with RFC0821.

If there is an "Updated by:" link, open a new tab for that protocol extension/clarification document and proceed to follow these same steps for that document.

If there is an "Obsoleted by:" link, follow that to a newer version of the same spec.

Lather. Rinse. Repeat.

Until the latest version of all documents have been found and the latest versions of all updates to those specifications are found.

Then google search for more SMTP extensions and implement the ones you are interested in.

jstedfast
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