What is the proper way to address an envelope to an alum and his wife?

Take John Smith as a sample name. If you are sure he is married, your best option is

Mr. & Mrs. John Smith

If this is for a reunion-type event, and you are not sure if the alum is married:

Mr. John Smith & Guest

For an alum who is a woman, if you don't know her new last name, you might try

Ms. Jessica Smith & Spouse (or & Guest)