1. SHOULD means, do it if you can/you have to have a really good reason if you don't do it. The only reason it's SHOULD and not MUST is backward compatibility. Mostly in context of "personal send mails", i.e. not automated mails. (For automated mail sending the expectations of you running somewhat up to date software is much higher).
2. You can't really implement mail stuff just based on RFCs:
- There docent overlapping RFCs which can sometimes influence each other and many of them obsolete older versions why others still relevant RFCs reference this older versions. This makes it hard to even know what actually is required/recommendation.
- Then you have a lot of "irrelevant" parts, which where standardized but are hardly supported/if at all. You probably should somewhat support them as recipient but should never produce them as sender today (mostly stuff related to pro-"everything is utf8" days). Like in general the ideas of "how mail should probably work" in old RFCs and "how it is done IRL today" are in some aspects _very_ far away.
- Lastly RFCs are not sufficient by themself. They don't cover large parts of the system for "spam detection/suspicious mail rejection". So it's a must have to go to the support pages of all large mail providers and read through what they expect of mails. And "automated mails need a message id" is a pretty common requirement. In addition you have to e.g. make sure the domain you use isn't black listed (e.g. due to behavior of a previous user), and that your servers IP addresses aren't black listed (they never should be black listed long term, but happens anyway, and e.g. MS has based on very questionable excuses "conveniently" black listed smaller local data center competition while also being one of the most widely used providers for commercial mail in that area).