In formal e-mail: Сидоров Иван Петрович (for executives patronymic is a must). or Иван Сидоров (usually if the position is not very high).
 In informal e-mail: Иван Сидоров. If you really want to sound formal, use patronymic. If you are writing to your friend, just Иван.

 In Russia in 99% cases it's a must to use the first name and the patronymic while addressing your boss or a senior. An exception can be if my boss is my friend or classmate, in which case i'd call him/her just using the first name.