Formal Letter: what name is used.. first or last?

Hi people..

In a formal e-mail, what name of the receiver should be used, the first or the last?

In my letter, I'm going to start it with "Dear Proffesor.......",
If his first name is Ihab, family name: Tameme, how should I use it?

And what about the inverse, I mean when the receiver is a student:
Dear ...... (first or family name)??

Thanks in advance..

We have a minefield here.

In British and American usage it is safest, in the first email, to address people you have not met by their title and surname/last name/family name, especially if they are in a higher position than you are -

Dear Mr/Mrs/Ms/Miss/Dr/Professor(One f, two esses) /Senator/General Tameme.

If you have met them, then address them in the way in which you have spoken to them.

If you are writing to a student/employee/ someone who is in a lower position than you, it is normally acceptable to use the first name - Dear Ihab.

If you are employed, find out what company/university/institution policy is.

If in doubt, go for title + last name. You may be considered a little old-fashioned, but at least you won't be considered rude.