Red Question Mark and Sender Name In Mail App

Hello Support Community,

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.

If i click on the question mark I get directed to the settings app to "Trust" the certificate for the sender.

The sender is legitimate and using a @Mac.com email address. I have just never seen the sender name turn red in the Mail app. Does anyone know what this means, or can anyone cite an article where apple explains the cause for this?