I got this error when sending.
xxxx@XXx.com on 9/1/2013
you do not have permission to send to this recipient. for assistance, contact your system administrator.
<owa.mydomain.com #5.7.1 smtp; 550 5.7.1:127.0.0.5:Client host 126.96.36.199 blocked using 88.blocklist.zap; Mail from IP banned. To request removal from this list please forward this message to email@example.com