You are confused.

The RFC is a request for comments. The specific one in question is about Internet Mail.

If server implementers want their mail to be delivered these are things they SHOULD do.

That's it.

It isn't something you can give to your lawyer, and nobody cares about your opinion about what you think "should" means you can make someone else do. This is how it is.

You are confused about what I'm doing. I'm not telling anyone what to do. I'm saying what category their actions fall into.

And the line of yours I quoted is still not supported by anything.