Seriously, I hope that none of the posters upthread arguing about SHOULD v/s MUST in some standards body document are in charge of real world money making software.

Google and Microsoft's email practices define a pseudo-RFC in practice. As an engineer, I hate this. As a civic participant I can vote against it. But as a person that sells my software services for a living, I am going to implement the Google/Microsoft standards to the letter, not argue about definitions in an RFC.