A while back it was asked if a title (e.g. Mr./Ms.) should be used when e-mailing a prospective employer. The accepted answer stated that, at least in the United States, it's best to err on the side of conservatism and use a title until you get a feel for whether or not the are okay with you addressing them by their first name.
This is all well and good, but what happens if you can't determine if the prospective employer is a man or woman because their name is unisex? For instance, imagine getting a followup e-mail from "Sam Doe". The name "Sam" can be short for either the masculine "Samuel" or the feminine "Samantha". As such, I don't know if "Sam" is a man or a woman (and because this is an e-mail, the name is all I have), thus I would not know whether I should address them as Mr. Doe or Ms. Doe.
How should I address this person in a reply? Should I guess the gender of the person and use a title (e.g. addressing them as Mr. Doe and hoping that they will correct me if the person is a woman) or should I just not use a title and address them by their first name until I know?
For the sake of simplicity, let's assume that this is in the United States and that the person is either a man who is alright being called "Mr." or a woman is who alright being called "Ms.", thus the only issue is the fact that their name doesn't indicate which of the two they are.