How to address email to a professor who prefers informality? How should I address an email to a professor who hates being called "Professor ___" or "Dr. ____?" He has made it very clear to students that he prefers to be referred to by his first name in person, but I have been told time and time again that one should be more formal when writing emails. Is this always true, or should I simply follow how he prefers to be addressed in person?
If the professor explicitly says not to call him by his last name, then don't. It would actually be more impolite to ignore his request in order to conform to some abstract "formality" rules. 

However, even if you are addressing your professor by his first name, you can still preserve some degree of formality. Some common norms include: 


do not use slang
avoid excessive contractions (e.g. use have not instead of haven't)
use your full name in the signature
be polite


In general, the rule of thumb is to be respectful and professional in your communications. Sometimes that includes addressing the person as "Professor X" or "Dr. X", but it does not have to. Just make sure you sound polite!