There are a number of ways that messages might go to trash, so please bear with me while I try to describe some of them. First - the email provider system that you use may have a "trash" folder, so those messages might be moved to Trash at the online mail end or once they download to Thunderbird. If they were in Trash already when you opened Thunderbird, they likely were sent there before Thunderbird ever downloaded them. (Your online email may call it Bin or Deleted or something else in which case it is easier to tell whether the move happened online.) Next if they were in another folder and you saw them there in Thunderbird, and later they went to Trash, and your email is IMAP (not POP) that could still be your online provider automatically processing them to Trash because of an online email setting. In either the online system or Thunderbird: * There may be filters set to act on certain messages from inbox, which might result in deleting them. * If you had selected a number of messages or folders and pressed the Delete key (you might accidentally select multiple messages with [ctrl]A or [shift]click). * You might accidentally drag and drop a group of messages to Trash. There are also server account/webmail and Thunderbird account settings that might delete messages after a certain length of time, or add-ons that 'tidy up' for you based on a timespan. I use those to keep certain folders or types of message for a limited time, and then get rid of them rather than manually needing to tidy my email folders. So, there are a lot of ways, mostly meant to be helpful and easy to use, but they can result in unintended deletion. There may be more I haven't covered too.