I’ve seen some discussion around magister with the short form mage and the abbreviation mg. Allegedly both ms and mr come from the Latin root that magister comes from, so lexically it makes sense.

No one is going to start a professional email “Dear mage” and be taken seriously, no matter how linked the etymology.

I suppose just avoiding gender and professionalism altogether is better though. Instead of “Hello sir,” a polite “Good afternoon” could suffice.

“To whom it may concern…”