Phish Threat Training Reminders

I would like to create a Exchange mailflow rule to add a banner to incoming Training reminders. Users are reporting those reminders to Sophos or ignore them because we are putting a "mail from external origin" banner in them. The mail is (default)  a lot like a phishing mail itself and therefore treated as such. 

If we could put a banner on them to inform the user it's legit. this would help.

Which Field in the Header could be used to make a 100% save mailflow rule. With other words is the an unique identification to verify this "mail"is coming from Sophos?

Just using the mail form adress doesn't feel save.