You need to make the following changes to your outgoing mail erver settings:
Change the outgoing mail server name to: cpanel02.avantihosting.com.au
Tick the box 'My server requires authentication'
The user name and password is the same as for the incoming mail server.
Open the "More settings" window and click on the advanced tab.
On the advanced tab select "SSL" (or "TLS" if "SSL" is not an option) for the outgoing mailserver. Also, change the port for the outgoing mailserver from 25 to 465.
Now click on "Apply" or "Ok" on all the settings windows and close down Outlook. After a minute re-open Outlook and try again. Sending emails should now work again.