Changing your Domain in Exhange 5.5

I recently lost my .com domain and registered a .ca instead. So I set about to change my exchange to use this .ca domain name. To give you backgound we had both the .com and the .on.ca domain and we were recieving mail to both no problems till we lost the .com. So now we can still recieve if you send to the .on.ca address but not to the new .ca
If I connect to the exchange on port 25 I get the following error when I do the rcpt to line:
553 Requsted action not taken: mailbox name not allowed

How can I fix this everything looks right I just don’t see the problem.