When sending mail via OMNIconnect's mail server (mail.omni.net.au) from another ISP, SMTP Server Authentication is required.

This usually can be found under the SMTP setting in your mail client or email application. For example, in MS Outlook:

Select the servers table and place a tick in the checkbox next to "My Server Requires Authentication".

In addition, some ISP's block port 25 to send mail to other mail servers, so port 587 is required to send mail. 

To change this in MS Outlook, click on the Advanced tab and in the Outgoing Mail (SMTP) box, change the 25 to 587.