I think the reason SMTP is hard to get right is not because of the many RFCs. The reason is that it's not documented.
Operational experience at scale is needed to know how to write an effective SMTP implementation, and that experience is half-documented by many people in many different information silos.
But... I'd also say it's an extremely forgiving protocol. In fact, it's the fact that it's so forgiving which makes operational experience required to implement it. A "correct" SMTP implementation has a lot of latitude in the choices it makes - and it's that latitude which makes life difficult.
Operational experience at scale is needed to know how to write an effective SMTP implementation, and that experience is half-documented by many people in many different information silos.
But... I'd also say it's an extremely forgiving protocol. In fact, it's the fact that it's so forgiving which makes operational experience required to implement it. A "correct" SMTP implementation has a lot of latitude in the choices it makes - and it's that latitude which makes life difficult.