Why do I have to use my ISP's Outgoing mailserver?

You don't have to use it, however, your ISP's mailserver should transfer your outgoing messages faster, than connecting to ours, because you will already be connected to your ISP network and the connection for sending mail this way is local.

I can't use my ISP's Outgoing mailserver. What is yours and can I use it?

Yes, we provide SMTP Outgoing server for our e-mail clients. The server hostname is smtp.yourdomain.com.au . If you are going to use this server, please be advised that you must enable the option "Outgoing/SMTP server requires authentication". Use the same settings as your incoming mailserver and you should be successful in sending mail.