Thunderbird is just a program sending emails, it is the server that is rejecting the emails. The server is generating the error message back to Thunderbird so you can see why server is not allowing your emails. Testing via webmail will not provide any answer because you are not sending via smtp. I'm assuming you are using the correct smtp settings. Outgoing server SMTP details * Outgoing server address: smtp.telstra.com * IF port: 465 * USE: Connection SEcurity: SSL/TLS * IF port: 587 * USE: Connection SEcurity: STARTTLS * Authentication Method: Normal Password * Username: Your Telstra email address - ending in ‘@bigpond.com’, ‘@bigpond.net.au’ or ‘@telstra.com.’ * Password: Your Telstra email account password - REMEMBER it’s case sensitive. But the issue does not state your settings are wrong. The Server spam filter has been triggered for your emails. I've had a look on internet and discovered this is a common occurance with Telsta and can occur using any third party email client. Rejection could be due to IP address so check the following most common reasons: You can check to see if your IP address or email address has been blacklisted: * https://check.spamhaus.org/ Some people found that if they unplugged the router/hub for about 10 minutes and then switched it on again, then restarted their email client all started working again as a new IP address had been reset. If you have recently enabled a VPN then this may trigger a Spam rejection because it is not using your IP address, so please check to see if you have a VPN enabled and switch it off. Many Anti-Virus products now offer a VPN. Maybe the Anti-Virus got updated and it auto enabled a VPN. I've come across people who discovered it had been auto enabled and it was causing a problem.