Earlier quoted context omitted.
> "I don't want to" is not a valid excuse for the client . If you're implementing a server, "the client SHOULD but didn't" isn't a valid excuse to reject a client either. You can do it anyway, you might even have good reasons for it, but then you sure don't get to point at the RFC and call the client broken.
> isn't a valid excuse to reject a client either. Yes it absolutely is: https://www.rfc-editor.org/rfc/rfc2119 is quite clear. 3. SHOULD This word, or the adjective "RECOMMENDED", mean that there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course. If the client SHOULD do something and do…
How does Google know whether or not the sender has a valid reason? They cannot know that so for them to reject an email for it means they would reject emails that have valid reasons as well.