How do you address someone in a letter if you have their first and last name yet do not know if they are male or female?
Address it to Mr - you will find out soon enough. Alternatively, phone the company [if it is someone in a company] and ask if the person should be addressed as Mr or Ms.
Personally I dislike the increasing habit of using the full name, but that of course is the current trend> Dear Les Blank. In some cases, the spelling of the given name should indicate sex.