You’re writing to someone, but don’t know if it’s a man or a woman.
This is something I had several weeks ago. I actually googled the person’s name and found him and so knew how to address him correctly!
But what if you can’t find the person?
It’s quite acceptable to use the full name. So address your email or letter to Dear Hayden Christ.
But you might also like to try the GenderChecker tool beforehand.