Untangle causing the emails rejected

Today we had an issue where emails inside the network was working but sending from outside was rejected. Tried everything still didn't work but once we restarted untangle all worked fine. Why would untangle cause that and give the following error

Delivery has failed to these recipients or groups:

user@xxxx.com.au
A communication failure occurred during the delivery of this message. Please try to resend the message later. If the problem continues, contact your helpdesk.


The following organization rejected your message: mail.xxxx.com.au.

Thank you.