I generally find that old accounts like this (I too have a lineone account, and have had tiscali too

I generally find that old accounts like this (I too have a lineone account, and have had tiscali too) work fine using your current ISP's SMTP server.

Often an ISP "authenticates" you as a bone fide user via your login to their own servers; so your connection to the internet, when it was provided by lineone, would have automatically authorised you to use the lineone smtp server. Now you're a bt.com customer, lineone don't recognise you.

I think that you'll find that you can select the bt.com (openreach? btinternet?) server in your lineone account's SMTP setting. DON'T change the incoming (POP/IMAP settings) though!

Tools|Account Settings; select the lineone account. Change "Outgoing server (SMTP)" to your btinternet SMTP server.