How to address a professor in letter?
When writing letter to an academic professor (not necessarily from student to professor), what is the appropriate way to address his/her title?
I have seen in letters using
Dear Prof. Xeven when the addressee is not a full professor
Dear Dr. Xthough, knowing that the addressee is a full professor
Dear Mr/Mrs Xthough, knowing that s/he has an academic title
All faculty (in the US) can be properly addressed as "Prof. X". We faculty are all professors, even though we are not all Professors.
I used to have an instructor who specifically didn't want to be addressed as "professor" because he wasn't a professor. He asked us to just call him by his first name. Though clearly I wouldn't suggest doing that unless they explicitly tell you to.
It also depends on the campus culture -- I went to St. John's in Annapolis, and the faculty were all called Tutors, and the *only* one we called Dr. was also a cardiologist. Everyone else (including students) was Mr./Ms.
The appropriate way to address someone is with their proper title. In your question, it seems you know what the title should/should not be. If you know, use it. If you don't know, it's generally safer to err on the side of formality.
While I personally prefer (and request) everyone to refer to me by my given name, I do feel it a little strange when someone I do not know / have never met addresses me in writing by my given name. Perhaps I am old fashioned but I expect introductions to be formal (and better to be too formal than too familiar) and then quickly get to preferred ways of addressing (i.e., to use my given name).
When corresponding (in writing or electronically) I would look to the signature. If they wrote:
Dear Professor Schmoe: Blah blah blah Sincerely, John ----- Dr. John Doe Agri-science Department University of Whatchamacallit
Then I would write back
Dear John: Thank you for your letter. Blah blah blah. Sincerely, Joe
I follow exactly the same rules. The only liberty I allow myself sometimes is to write "Prof. Schmoe" instead of "Professor Schmoe" in the very first letter if I am sure that he knows of me and wouldn't mind to communicate with me informally. However, I never start with "Dear John" or abbreviate in any way in an "official" message (like job application, etc.) that may be filed with the office rather than read in private even if the addressee is among my personal friends. Also it is better to make an error of addressing a secretary as Prof. than that of addressing a professor as Mr.
@earhling, at this point (i.e. after the original email and your reply, where you used a more informal `Dear John`) would you consider appropriate if John wrote back starting with `Dear Joe, thanks for...` or it's still better to use again `Dear Professor Schmoe, thanks for...`?
@andrea In the example in my answer, where I signed my letter "Joe" then I consider it completely acceptable for him to respond "Dear Joe" - however, to be even more clear, I _could_ add "Please call me Joe." However, since I've signed the letter "Joe" I would expect him to interpret that as an invitation to use my first name from here forward. If he continues to use "Professor Schmoe" I would think either he was a bit too formal or he didn't read the signal (my signature) clearly enough.
FWIW this is the convention I go by as well. If someone signs a letter or email with their first name only, that is an implicit invitation to address them by their first name in any future written communication. If they sign it with their first and last name, or their title and last name, that indicates you should stick to formal forms of address. I think this convention is widely known about.