TalkTalk require email sending to use port 587 and the StartTLS encryption command to use TLS 1.2 encryption and outgoing mail authenticated by username (the full email address) and password of the sending TalkTalk email account.
Previously, you were using port 25 to send outgoing mail and no password authentication. If sending from a personal domain address you must use the outgoing SMTP servers of the personal domain hosting service not the TalkTalk servers.
The SMTP outgoing server must be linked to the TalkTalk email address you're sending from. The SMTP outgoing server settings obviously have a username and password for a different email account.
Because each email address needs its own outgoing SMTP server setup you can add an SMTP server in the Outgoing SMTP server list.
Then on the Account Settings page, select the email account in the left pane and you'll see at the bottom of the main screen the ability to see and select from the drop down list the outgoing SMTP server settings for that email account.
The description needs to identify the (From) TalkTalk email account
Server name: smtp.talktalk.net
Port: 587
Connection security: STARTTLS
Authentication method: Normal Password
User name: The full email address for the email account
Having linked each email account to dedicated outgoing server settings, when you send mail you'll be asked for the password. Tick to remember the password before entering the correct password.
If you don't wish to edit and set up the SMTP server to match the sending email account then just delete the email account from Thunderbird (if IMAP or backup the emails if POP3 and then delete) and start afresh to set up a new account.
