Having used Thunderbird for some time, I installed it on a new PC, and found that virtually all my emails on a particular account seem to have been deleted, despite the app being set up to retain them. I checked on the webmail (BT) and they have gone. The account is set up as IMAP - as is another account on the same server, which seems OK. BT won't engage with me on this as they "don't support third party apps" I assume that 3 years worth of important client history is gone for ever? Can anyone suggest what I did wrong, or is this a software failure?