How do you address a letter to a past Ambassador?

Ambassador William Smith,

00000 Chelsea Place,

Seattle, Washington U.S.A.

0000000 (area code #) Dear Ambassador Smith: NOTE: Keep everything precise and to the point (easy reading) when writing ANYONE in an important position (even business.) People are busy and they want to read your letter with a point being made and if you can have a resolve to the problem (you don't always have too) but it's appreciated.