I have two email accounts from different suppliers (TalkTalk and gmail), and this happens with both. I had sent an email, and I opened it in Sent Items to re-read it, and there was a full stop where there should not have been, so absent-mindedly, I highlighted it and deleted it - and found that the entire email had been deleted from my Sent Items. Unexpected, and not very friendly! If it's going to delete the mail, couldn't it do what software usually does - "Do you really want to delete this email?" This was an important mail, concerned with the sale of my house, and I wanted to keep a careful record of everything concerned with the transaction. In an effort to recover it, I read that it can be recovered as long as the folder has not been compacted, but Thunderbird hassles me continually to compact folders, so between deleting the mail and reading the method to recover it, I had been prompted, and had compacted the folder. This is not the first time this fault has bitten me. The first time it happened, I did not understand why. I thought I knew what I had done, but I couldn't believe the software would be so user-unfriendly. The second time confirmed it, and I later tested it with some old, unimportant mails, and verified that this happens every time, regardless of the email account. I have only tested it in the Sent Items list, not the Inbox or any other folder. IMHO, this is quite an ugly fault that you may want to address? My Thunderbird version is 52.9.0 (32-bit)