First: Check your Firewall Exit Thunderbird if running. Access Firewall, which may the default that came with OS or it might be controlled by your Ant-Virus.

Thunderbird must be listed as an allowed program.

If it is already listed as allowed - then block it and ok it. Then Access Firewall again and set up Thunderbird program as allowed.

I have known a Firewall to get confused over Thunderbird after an update. It thinks Thunderbird update is a different program, so resetting can fix.

Then check the current server settings to ensure all is correct. https://www.telstra.com.au/support/email/setup/imap-pop-smtp-mail-server-settings

Incoming server name: imap.telstra.com Port: 993 Connection Security: SSL/TLS Authentication Method: Normal Password User name: full email address

Outgoing SMTP server name: smtp.telstra.com SMTP Port: 465 (recommended) Connection Security: SSL/TLS Authentication Method: Normal Password User name: full email address

Alternative SMTP port: 587 If using port 587 then use Connection Security: STARTTLS

There seems to have some issues in last 24 hours - see https://downdetector.com.au/status/telstra/

Check to see if service has issue: https://outages.telstra.com.au/#/