Typically, MUST means that if you don't do that then something will break at the protocol level.

SHOULD means that if you don't that, bad things are likely to happen, but it will not immediately break at the protocol level and during discussion in the IETF some people thought there could be valid reasons to violate the SHOULD.

Typically, IETF standards track RFCs consider the immediate effects of the protocol but often do not consider operational reality very well.

Sometimes operational reality is that a MUST gets violated because the standard is just wrong. Sometimes a SHOULD becomes required, etc.

Certainly for email, there is a lot you have to do to get your email accepted that is not spelled out in the RFCs.