SHOULD generally means: some people might require it. implement it for best results

backward compatibility makes it hard to add MUST. using SHOULD is a good alternative

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.

MUST means omission is unacceptable. SHOULD means MUST unless you have a good, well-reasoned excuse.

I SHOULD have 8 hours of sleep every night. It's RECOMMENDED. However, there are times where it's best I don't (e.g. because of work, or travel, or needing to take someone to the hospital, etc.). It's definitely not that I MUST sleep 8 hours every night.

“When jump getting over a wall, you SHOULD use three points of contact.”

For most cases you should use three points of contact. However, there may be other situations for example if someone is giving you a leg up, or you can pole vault, where another solution is preferred.