One thing I've learned to do is that when I address someone in a letter or email who I don't know, I address them by their first name, as opposed to last name. Had an experience a while back where I emailed someone with the first name Connie and addressed it to "Mrs. Lastname." Turns out this Connie was a dude (like an old school baseball manager), so he responded back to me with a butthurt "Mrs. SayHeyDeacLastname" (my first name is def not a girl's name, so it was intentional). So, from then on I send stuff addressed to people's first names, unless I'm positive of the recipient's sex. May be less formal, but I'd rather be less formal that get their sex wrong.