alt Hacker News> SHOULD is a requirement.
I once had a job where reading standards documents was my bread and butter.
SHOULD is not a requirement. It is a recommendation. For requirements they use SHALL.
My team was writing code that was safety related. Bad bugs could mean lives lost. We happily ignored a lot of SHOULDs and were open about it. We did it not because we had a good reason, but because it was convenient. We never justified it. Before our code could be released, everything was audited by a 3rd party auditor.
It's totally fine to ignore SHOULD.
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Yes, except there seems to be a move on the best words from SHALL to MUST and from SHOULD to MAY. IANAL but I recall reading this in e.g. legal language guidance sites.
Maybe the standards documents you are used to differ from RFCs, but here is the official language:
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.
Email is about standards like browsers were about standards in 2017...