If you share screenshots of the SMTP server settings pane, and the complete text of error messages, some suggestions may be available. thanks.

Here are some screenshots which will hopefully be of some help. This has happened with Thunderbird twice before on a different laptop.

Try 587 and STARTTLS . also, did you try entering password? I know little of btinternet, so I may not have a solution.

BT recommends port 465, SSL/TLS, so you have the correct settings.

Confirm the password works for webmail, and disable a VPN if one is used. Removing the entries for the account in Settings/Privacy & Security, Saved Passwords, restarting TB and entering the working password, might help.

Are both of your accounts with BT, and if so, do you have separate smtp servers for each one, differing only in the User Name?

Thanks for your suggestions. Both passwords work on webmail and are both with BT. I think the outgoing server is the same for both. I have a VPN but do not use it for email. The last time this happened and I removed the entries for both emails on TB, everything in both my TB inboxes was deleted, even though the instructions led me to believe they would remain. I cant allow that to happen again. Have tried to reconfigure just the outgoing server again just in case it needed a 'refresh' but TB will not allow that.

Check this topic to make sure your two smtp servers are defined and connected to the correct accounts:

https://support.mozilla.org/en-US/questions/1350593

Thank you again. For my circumstances it was a little simpler. In TB > Tools > Account Settings > Outgoing Server Settings > click chosen account in large frame > Edit > Authentication ....I simply removed the need for Authentication method. This may have been changed in a very recent AVG security update. I had no memory of how the settings were before the change.

The problem is almost certainly due to AVG. Disable Mail Shield, and any other AVG action on TB, and re-apply secure server settings, including authentication.

Thank you once again. When I returned to TB after the changes I mentioned above, they reverted to being blocked again. I have now Disabled only the Scan Outbound emails (SMTP) in Email Shield in AVG and all is now as it should be.