How to address a formal letter if unsure of gender?

[m.paris]

You might consider "Dear Morgan Smith:" It is much better than just the first name although it is not, strictly speaking, proper. As others have said, try to determine the gender if at all possible. Using "Dear Morgan" sounds like you are addressing a child or a close acquaintance.

EDIT: Dang, I need to speed up my typing. Someone beat me to it.