Bigpond have set up restrictions so that nothing outside their own network will have access to anything apart from their own mail server on port 25. ( outgoing mail server)
So if you are using multiple email accounts other than @bigpond.com Telstra Bigpond broadband connection, will not allow you to send email, unless you use Telstra Bigpond’s very own SMTP server, mail.bigpond.com
Bigpond call this “security”.. and their support staff are often not aware of this until you ask to have your problem escalated to a higher staffer.
Best advice is to avoid using Bigpond where possible.