From: DeskNow Knowledge Base
http://www.desknow.com/kb/

I get this error when sendin out emails: SMTPAddressFailedException: 554 : Relay access denied
http://www.desknow.com/kb/idx/11/111/article/
This error means that the server that DeskNow is sending to does not allow mail relay.

If this happens on every email sent outside, you're probably using another SMTP server as mail relay (your ISP?), and configured it in the Administration/SMTP Outgoing/SMTP forwarding section. Check with your ISP if you are authorized to relay, or try configuring DeskNow to use the username/password provided by your ISP for SMTP forwarding.

If this happens only on some email addresses, check with the administrator of the remote mail server.