"Re:" in a formal business letter

[Pechorin]

Hello.
Where does "Re:" go in a formal business letter? Shall it be before or after the greeting?:
A)
1. Address
2. Re: Contract No....
3. Dear Sirs:
We are pleased to inform you that...
B)
1. Address
2. Dear Sirs:
3. Re: Contract No....
We are pleased to inform you that...

Which word order is correct A) or B)
Thanks in advance.

[Thredder]

either. I've seen it both ways.