Pulling exact quotes out, SHOULD means "there may exist valid reasons in particular circumstances to ignore a particular item" while MAY means "an item is truly optional."

I don't think this can be interpreted as simply "should is optional".

I think that is a bit to easy. MAY is described ar optional.

SHOULD - Should really be there. It's not MUST, you can ignore it but do not come crying if your email is not delivered to some of your customers ! you should have though about that before.

> So should is optional, people!

Only once you have satisfied:

> but the full implications must be understood and carefully weighed before choosing a different course.

In other words, you better have a damn good reason for deciding not to do it.