Placement of re: in a business letter

[Unregistered]

In a business letter, I want to include the "re:" what the letter is about.
Where is it supposed to be placed - before or after the salutation?

Example
Date
Name, Address
Dear Sam:
Re: your account

or
Date
Name, Address
Re: your account
Dear Sam:

[sploke]

The first salutation is correct. After referencing, you then go on to the introductory paragraph..