There is an established protocol for addressing the man who occupies the office of POTUS. The addressing of the letter followed that protocol. Even those who do not believe Obama is Constitutionally eligible still address him as Mr. President. Its just proper etiquette.
If Adm Mullen refused an order holding up a verifiable birth certificate showing Obama was born in Timbuktu, YOU would still be claiming he holds the office legitimately, so let’s not have you playing silly games about how a letter was addressed.