Do you have each account sending through separate smtp servers, with matching User Names, or do they both send through a Default smtp?

https://support.mozilla.org/en-US/questions/1324013

both accounts have always been on smpt.talktalk.net as default

Each account can send on smtp.talktalk.net, but the User Name for the two smtp servers should be different, so account A sends on smtp.talktalk.net which has User Name A, and account B sends on smtp.talktalk.net which has User Name B.

will check with dad's computer t later to see if user names have been somehow altered/ duplicated

although i am not familiar with thunderbird, i find there are 2 different user names, but there seems to be a log in problem, where the saved passwords are not recognised when entered, & you get on loop trying to do so AB pp Ted

Remove the Saved Passwords from Options/Privacy & Security, then restart TB and enter them when prompted. Select each account in the left pane of Account Settings, then click Edit SMTP server in the right pane to confirm the smtp User Name (email address) matches the selected account.

Furthermore, note that timeouts are often caused by antivirus scanning of outgoing mail, a feature that should be disabled.

have worked thro your last notes, checked all settings were as i have always known them, but no joy only one of the accounts passwords appeared in the saved list. tried manual set up from add mail a/c still not working -see screenshot cannot see what error i am making. have spent many hours trying to fix repeat original request - can I start over wirh same addresses ?

Enter the settings given here:

https://community.talktalk.co.uk/t5/Articles/Email-settings-IMAP-amp-POP3/ta-p/2204399

incoming IMAP mail.talktalk.net on port 993, SSL/TLS, outgoing smtp.talktalk.net on 587, STARTTLS; authentication = normal password, User Name = email address for both servers.