It's about postfix not allowing the sender to send to the receiver - not being able to access ..something (Sorry if you already guessed that - not trying to be a smarta...).
Where do you get this message?
If you miss inspiration; I would start looking in /var/log/maillog and get a more full message. That might tell you what it IS the sender is not allowed to access. Then take a look through:
Those are fairly well documented inside, and you might be able to find why the user is not allowed to access that .. thing.