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

I cannot send mail using Outlook/Eudora/etc: 551 Relay denied
http://www.desknow.com/kb/idx/11/024/article/

If you're getting the '551 Relay denied' message when trying to send emails to external domains, you have two possible solutions:

  1. you can specify an IP address (like 192.168.10.13), or an IP subnet (like 192.168.10) to be allowed to send emails to external domains (relay). This is done in the Configuration wizard.
    Mail clients that are connecting to DeskNow from one of these addresses will be allowed to send emails outside. This is useful for instance if you want to allow all the computers in your office to send emails outside.
  2. You can configure your mail client to use SMTP authentication. For instance, in Outlook you can check the box that says 'My server requires authentication' in the 'Outgoing server' section of the account properties. Then specify your username and password. This is useful if you change IP address often, and so you can't rely on case 1.

Note: you can use both methods at the same time. Ex. method 1 for users in the office, and method 2 for mobile users.

Note: if a Cisco PIX firewall is located between your mail client and your mail server, chances are that its 'Mail Guard' functionality will be turned on. This will filter out any authentication command sent by the mail client, so SMTP Authentication will not work. It is recommended to disable this feature in the firewall if you want to enable SMTP authentication.

As a non-authoritative guide, here are the 2 possible commands to disable it.  They are to be issued once in a config mode (#: config t)
 
1) if behind a PIX firewall (running Firewall OS):
    no fixup protocol smtp 25

2) if behind a Router OS based device (Cisco 831, SOHO 90, etc)
no ip inspect name DEFAULT100 smtp   
(be sure to check to ensure that "DEFAULT100" in the name of the base rule in place by issuing the "show run" command)