My Postfix server is rejecting mails sent from Blackberry Devices. The message blackberry users receive is as follows

You do not have permission to send to this recipient. For assistance, contact your system administrator.

< emthink.thinkahead.net #5.7.1 smtp; 550 5.7.1 Message content rejected, id=32589 - BANNED: P=p003,L=1,M=multipart/mixed | P=p002,L=1/2,M=application/ms-tnef,T=tnef,N=winmail.dat>

Advanced Thanks for your help .