[sigseg1v]

It's not required but they need to understand the implications. In this case the implication is that Google drops the mail. So clearly they didn't understand the implications.

[lukan]

Yes, but this might be googles fault for not respecting/missinterpretating the spec.

[dodobirdlord]

Google probably did parse these messages as well-formed before inspecting them and deciding to drop them based on the lack of this field. The RFC imposes no mandatory obligation to deliver messages just because they are well-formed.

[mort96]

So you agree that calling it a requirement is wrong?