POP only sees Inbox. IMAP syncs with any folders you subscribe to on the providers server

Bugger! I am using 'BT Yahoo Mail' and it appears that it is POP. Is there a way for a user to change this to IMAP, or is it up to the service provider?

You can't edit a POP account to make it IMAP, but it appears BT supports IMAP accounts:

http://kb.mozillazine.org/Convert_a_POP_account_to_a_IMAP_account

http://bt.custhelp.com/app/answers/detail/a_id/44917/kw/bt%20mail/c/346,6588,6591

Use Connection security = SSL/TLS and Authentication method = normal password.

That's brilliant! Thank you. Now, one final question - if I change the settings on ThunderBird to reflect those for BT's IMAP settings, what effect will it have (if any) on existing emails stored on ThunderBird?

Before you delete the POP account and add it as IMAP, copy the existing messages to folders created under Local Folders, e.g. Old Inbox POP and Old Sent POP.

Just installed Thunderbird. I do have IMAP, but the program only sees my Inbox, not the other folders... How do I get them in the folders view?