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In a completely different field, navigating ships at sea, the Collision Regulations which define how people must conduct ships at sea, they use the words "Shall" and "May" to differentiate legal requirements and what may just be best practice. "Should" intuitively means something more like "May" to me

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Happily, the meanings in RFCs are clearly specified, see https://www.rfc-editor.org/rfc/rfc2119.

Note "the full implications must be understood and carefully weighed before choosing a different course". Gmail and the other big hosters have full-time spam teams who spend a lot of time weighing implications, so I assume the implications of this was weighed.

And EVERY rfc has a paragraph talking about rfc 2119 in the preamble.

I guess that's why nobody reads it. /s

I feel like "SHOULD" and "SHOULD NOT" are redundant. You end up having to assume someone else treated them as a "MAY". If you control all the endpoints in a private implementation you can just deviate from the standard & not implement a MUST, it's your private implementation. There's thus no difference in public implementations between "SHOULD" and "MAY", and no difference in internal implementations between any of the words. They are therefore redundant, requirements are either mandatory or optional, there's no middle.

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