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

The content filter for outgoing messages works only when sending from the webmail, not from a mail client like Outlook
http://www.desknow.com/kb/idx/4/166/article/

DeskNow can apply different content filters for every domain.

This means that in order to determine which filter to apply to an outgoing message, it needs to determine the sender's domain.

When the email arrives from a mail client, the only way to be sure of the identity of the sender is if the mail client uses SMTP authentication to send - the From header can be easily forged.

Solution

Make sure to enable SMTP authentication in the mail client. In this way, DeskNow can determine the identity of the sender and apply the appropriate outgoing filter for that domain.

To force users to use SMTP authentication, simply disable any IP-based mail relay.