In Colons^x, Salutation^x

Really? Honestly, I've worked in English offices for nearly 20 years and I can't recall seeing a colon used after a salutation. It may be one of those 'correct in theory' things that no one actually does in practice?

It is still correct to use a colon after the salutation in a business letter.

Hello from Japan. I am writing a business letter addressed to "Her Excellency Ms. (First name) (Surname)", and wondering what to do with the salutation. Should I write "Dear Madam:", "Dear Her Excellency Ms. (Surname):", or just "Madam:"? Also, should I use a colon or a comma?