Is it proper to address a person in the address block as Mr. So & So if typing i.e. CPA following his name?
Yes. You would say something like "Mr. Robert Jones, CPA." If the person is a doctor, however, you either use the honorific "doctor" and omit his title, or type his name followed by his title, such as:
Marcia Johnson, M.D. -or-
Dr. Marica Johnson
You never put "Dr. Marcia Johnson, M.D."