Question about formal/informal letters to college professors/instructors...

<p>Okay, here's the deal. I've been sending formal (Dear... Sincerely) letters to a pretty young college instructor (he is still in school working a degree, not married) of mine and he has been sending informal emails back to me (No heading, closure). Should I keep sending him formal emails or revert to informal and conversational messages? =P I don't know because it just seems weird... I think it would be weird if I switched though?</p>