Your internet service provider may not allow the use of third party email servers for outbound email. (e.g. Telstra, Optus.)
How to fix:
Call your internet provider to ask how to set up your outbound email settings only. This is not ideal as it can cause SPF issues with email being blocked by spam filters.
Alternatively you can use an external email host by entering their MX records to the domain settings with your Ashop admin panel.
