RichPie and Tej2020 are both right. Thanks for those posts.
I also found though that some of the older talk21 accounts no matter if you follow those instructions still don't validate the username.
Solution:
Login to your BT Yahoo email account with your
firstname.lastname@talk21.com username and password.
Go to settings and forward your mail (temporarily) to another email account.
You will get a verification email in that account for security.
You'll probably see that the username is not
firstname.lastname@talk21.com but
firstname.lastname.t21@btinternet.com
Now here's the trick.
Go back to your BT Yahoo email.
Don't forget to disable the forwarding of email (otherwise you'll never get any new emails in this account).
Logout.
Now try logging in as
firstname.lastname.t21@btinternet.com and your usual password.
Same account right?
Follow RichPie's instructions (below) for allowing the email to be POP'ed.
1./ Log into you talk21.com account:
options
Mail options
POP3 & Forwarding
click on "allow forwarding"
Save.
Now in your phone setup your email with
Setting up the mail use these:
Incoming Mail (POP3) Server - mail.btinternet.com
Port - 110
Outgoing Mail (SMTP) Server - mail.btinternet.com
Port - 25
and crucially, the username and password must be
firstname.lastname.t21@btinternet.com with your password as this seems to be a migrated username/password combination on the migration of the talk21 accounts into Yahoo and is a popp'able account.
This should work.