Question : #5.7.1 smtp;550 5.7.1 Unable to relay

In the past two weeks, we have been getting NDRs on some email addresses:

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


I've looked into MS' KB and could not find any solutions.  Any ideas?  

We're using exchange 2K, SP3 on Win2K Serv SP3.

Thanks in advance.

Answer : #5.7.1 smtp;550 5.7.1 Unable to relay

sounds to me like you've been blacklisted and your recipients are using a RBL to block any sender on their blacklist.