Red Question Mark and Sender Name In Mail App

I came across the same issue as in this earlier report (which I can no longer answer because Apple timed it out, so I am forced to create a new "question" and answer myself).

I recently received an email to my corporate email account, and when i opened the email the senders name was red with a red question mark next to it.

Apparently, this is iOS Mail's "Lassie-language" attempt to tell you something that macOS mail shares with you freely:

It was a cryptographically-signed message that was signed badly. On iOS Mail, this header just shows up as the sender's name in bright red, with a red question mark, and no other explanation.