Another subtlety is an explicit and implicit addressing style unification among unequal participants.
I. [explicit] When you are referring to superior, using the aforementioned "f.n.+patronymic", he is expected to return you in the same way, even (or especially...) in the case of student-professor interaction.
II. [implicit] You are still expected to use formal addressing, when addressed informally by superior - but when that expectation holds, it is actually considered unacceptable, or bad style at least, for superior to use such addressing; unless you're in the student-professor relationship with the superior, or you're working in some shitty-unwitty paperstocking dogfood reselling organization, or you're unskilled/simple-skilled labour in a government institution. Rephrasing, superior should make sure that you both use the same addressing style (ie. as in I.).