Dear..

How do you start a letter when you don't know the person's gender?

Thanks W. , I just wanted to show to some colleagues- teachers how wonderful this site is. A bit late , though.:);)

If it's an email I usually use hi. What about you?

from Pocket Fowler's Modern English Usage.
Dear , as part of a formal greeting at the beginning of a letter, was introduced in the 15c in various formulas. Dear Sir and Dear Madam have become the
most formal types, with Dear Mr Jones and Dear Mrs/Miss/Ms Jones serving as more socially neutral alternatives. Increasingly people are using the full
name as in Dear John Smith or Dear Jane Smith, or (especially in circular letters) a descriptive name as in Dear Customer or Dear Colleague, in order to
avoid the need for a title, which is a welcome tendency.
Personally, I’ve noticed these trends:
1 In business communication, folk prefer open punctuation. Keep the ‘Dear….’ But remove the comma and keep everything tight to the left margin
2 regarding email, the dear or even ‘hi’ is dropped: a heading, the body of the message and a signature plate
3 If I was really following this for a reason, I’d use a style manual appropriate to my region