(now that the question is finally on-topic, I am happy to contribute my 2 yens ;-)
The general use of 先生 (sensei) when addressing a professor/doctor/etc. is already discussed elsewhere on JLU... As for the particular case of writing to someone who is your peer (in rank and range of age), the answer is:
No you do not have to use it.
My colleagues/bosses (themselves Drs. and/or Profs.) usually do not use 'sensei' (only 'san') when mentioning or talking to another Dr./Prof.
Of course, if the professor in question is some famous old professor (or simply an authority figure to the speaker), 'sensei' is used as a form of politeness.
The bottom line is that you use 'sensei' just the same as you would 'Prof.' in English: you would probably call your (possibly tenured) lab neighbour "John", but your advisor or some visiting professor will get "Prof. Smith".