Common Australian ISPs outgoing mail servers
Most hosting companies avoid acting as the outbound relay for their clients, even if they are hosting their email. Assuming the ISP offers a reliable outbound SMTP server, there's usually nothing to be gained in routing the mail via our servers - and incurring the traffic charges.
As the hosting provider, we're usually running the inbound email so we're often the first port of call when things go wrong. This list of outbound SMTP mail servers is useful when checking customers mail client configuration: