When writing a letter to an unknown recipient use: To Whom It May Concern.You may also use: Dear Sir/Madam if you know the sex of the recipient but not their name. If you know the name of the recipient the letter should start: Dear Mr/Mrs/Miss/Ms/Dr X.

A colon is traditionally used at the end of a formal greeting.

Example: To Whom It May Concern:

If you used To Whom It May Concern, custom dictates that you capitalize each word.