In his letters to Emperor Trajan, Pliny the Younger used the salutation

Gaius Plinius Traiano Imperatori

Interestingly enough, in Trajan's replies he's a bit less formal:

Traianus Plinio

To me this is still kind of impersonal and formal (and I would not advise calling your professor Imperator unless you're really trying to brown-nose), so I would use something like

Joe Johnson Magistro Smith, salve.

In his letters to Emperor Trajan, Pliny the Younger used the salutation

Gaius Plinius Traiano Imperatori

putting the sender (himself) in the nominative, and the recipient (Traianus) in the dative.

This is kind of impersonal and formal (and I would not advise calling your professor Imperator unless you're really trying to brown-nose). Interestingly enough, in Trajan's replies he's a bit less formal:

Traianus Plinio

Jerome's translation of Paul's Epistula ad Thessalonicenses Prima opens

Paulus, et Silvanus, et Timotheus ecclesiae Thessalonicensium in Deo Patre, et Domino Jesu Christo. Gratia vobis, et pax. (1 Thes. 1-2)

For an email to a professor, though (and a non-inspired work), you don't need to be quite that fancy. I personally would use something like

Joe Johnson Magistro Smith, salve.