I have contacted Telstra (my mail service provider), and because my web-mail still works, they say it is not their issue. They recommend I contact you for assistance. I can receive emails via Thunderbird but can't send any without this error message.. It's only happen today.

Sending of the message failed. An error occurred while sending mail: Outgoing server (SMTP) error. The server responded: Connection refused - OB115. 121.222.95.23 is blacklisted. If you believe this to be in error, please contact postmaster@bigpond.com.