I remember that at some point, many years, ago, I read RFCs 821 (on SMTP) and 822 (on the format of email messages).
Later, RFCs 2821 and 2822 were released, which updated and obsoleted 821 and 822 (respectively), and I read those at some point, too.
I was just made aware that these, too, have been obsoleted: by RFCs 5321 and 5322. Ideally, I'd like to read and digest those two, too, but I'm not sure when I'm going to (find|make) the time for that.
(And let me just say that it's a pity those drafts weren't ready when the 4800's were being released or that they couldn't wait for the 5800's; I liked the fact that the 2821/2822 numbers were so mnemonic relative to the original 821/822 ones. Sort of like how ISO 10646 sounds like ISO 646. Though at least the numbering is exactly 500 numbers "off", which is a bit easier to remember than if they had been numbered, say, 5476 and 5477.)