Question : #5.7.1 smtp;550 5.7.1 Unable to relay

I am currently getting this message when sending to an external mailbox.  I am using exchange 2003.  Is there something I can change so that the message goes through.

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

Answer : #5.7.1 smtp;550 5.7.1 Unable to relay

If you can send to all other domains correctly, then the problem is with the remote side.
They are probably blocking your messages. You need to ask them if that is the case.

As an interim measure, try routing email through your ISP for that domain, using an SMTP Connector.
http://www.amset.info/exchange/smtp-connector.asp

Simon.