When the Recipient is Unknown
If you have never met the recipient before, are unaware of his/her name, and/or don't know their gender, you need to be very careful as to how to salute them. The best bet is to write Dear Sir, Dear Madam, or Dear Sir or Madam. On the other hand, when the gender of the recipient is unknown, then go with the old time favorite To Whom It May Concern. This covers both the sexes and sounds very formal, and that's the tone you want in your letter.