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

550 MAIL FROM local domain, but you're not authorized to relay. Suspected address spoofing. Command rejected.
http://www.desknow.com/kb/idx/11/146/article/

This message is generated by DeskNow.

This message means that:

1. address spoofing blocking is enabled in the Administration/Antispam page

and

2. DeskNow received an email with a 'from' address of a local account, but the mail client was not connecting from a IP address authorized to relay, and was not using SMTP authentication.

In this case, the sender was not authorized to relay, and trying to send to a local address claiming to be a local user account. DeskNow blocked this message as this technique is typically used by spammers.

Solution

Implement one or more of the following:

1. make sure to configure your client to use SMTP authentication ('My outgoing server requires authentication'), and that SMTP authentication is enabled in DeskNow (it is by default).

2. connect from an IP address authorized to relay, or authorized mail relay for that IP address (see Administration/Mail/SMTP Incoming/Relay)

3. disable the blocking of address spoofing in Administration/Antispam