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.

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

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.

And (possibly even more to the point), you must decide not to do it, rather than simply throwing code at the wall until most emails make it through unscathed.