Can't Deliver to one Domian

[Lynden A. Philadelphia]

I have a SBS 2003; for some reason users in that office can's deliver to 
Sympatico.ca domain. we can deliver to any other domain without any issue.  I 
have checked exchange and the sonicwall an can not see anything.
 
I should also mention that Bell is the ISP in question and no black listing is 
in place.
 
Can some one help?
 
 
Lynden