My assumption is that when the antivirus system scans incoming emails and emails stored in Thunderbird folders it finds something unexpected, and treats the folder and contents as an 'Infected item' and removes it. I was surprised to discover that the email scanner only handles POP3 protocols.

In relation to my antivirus program, (Bitdefender) which is currently the suspected cause of the corrupted Thunderbird system, I had a look at the what thier email scanner did.

Note: all these folders worked as expected for a while before the problems started.

In relation to the 'Sent' folder, if I'm working on an email that relates to an earlier email and want to check something in the earlier message, then I do not want to stop and log in to my IMAP to do it. This is also true for the 'Junk' folder, because if Thunderbird mistakes a file as junk when it is'nt then currently that email will simply disappear and I will never know that it arrived. I need a'Drafts' folder because if I am generating an email on Thunderbird and stop half way through, that part finished email is never giong to be sent to my IMAP account. Consequently, if I accidently delete a file, it cannot be recovered. In relation to Thunderbird, the 'Deleted folder shows under 'Local Folders' but has stopped working and is empty. You are correct in saying that my 'Sent' emails are available on the IMAP account at as that folder is not set to empty automatically. My IMAP admin account, (on website), is set to delete emails 48 hours after they have been picked up and this elimates the problem of having to remember to manually delete messages before the 'daily' mail box is full and won't accept further email inputs. Not sure Iagree with your latest response.

Kls mail backup