Is this how it's setup now?
Talk Talk Email Settings
E-mail address: firstname.lastname@example.org
Reply-to address: email@example.com
Incoming mail Sever: mail.talktalk.net
Incoming mail account type: POP3
Incoming mail port: 110
Outgoing mail (SMTP) Server: smtp.talktalk.net
Log on using Secure Password Authentication (SPA): leave unchecked
Outgoing mail port: 25
Use SSL: leave unchecked
(They may have changed to SSL & different ports/authentication now).
The Password rejection can confuse people since it's a catch all meaning...
This Password, Username, Authentication method... is not recognized on this Port to this Server, more than one entry in Keychain for each in/out entry, or a server end problem.
If using a browser to login via WebMail works it's not Name or Password, but one of the other ones.
The receiving email ports are:
IMAP is port 143
IMAP-SSL is port 993
POP is port 110
POP-SSL is port 995
Outgoing ports are...
SMTP and SMTP-SSL is on ports 25, 587 and 465. Port 587 has to be SSL, and port 465 is enforced TLS-wrapped and is generally used by Outlook users.