A little more info, it could be something as simple as the IMC in 5.5 doesn’t allow forwarding rules.
When doing a migration, I try to keep it simple. In the new world, can you email gmail, do you get a response. Can you email to someone on the old system via SMTP addy?
You’re playing with forwarding rules and recipient policies in 2003 and the old IMC in 5.5 work differently and so what applies to one, may not apply to the other.