RFC language is expmicltly defined in 2119[0]. Any other interpretation is incorrect.
replyThank you for that. So should is optional, people!
replyI think that is a bit to easy. MAY is described ar optional.
replySHOULD - 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.