Last night I watched as a year's worth of emails disappeared before my eyes. I use Apple Mail app, no they aren't in another folder, yes it's on IMAP, no they aren't in Webmail either and yes, I got a woeful response from customer service this morning. Not even an apology. That's all my tax stuff and no guarantee of recovery. Why does this keep happening to people and why does BT think it is in anyway acceptable?!
If your emails are no longer on the BTMail servers when you check your email via webmail and they are not in any of the other folders that you have, it means that they have been permanently deleted and can not be recovered.
Going on previous posts and moderator intervention, the best that you can hope for from BT is that they may be able to tell you which device caused the email deletion.