Your Inbox mail fails possibly got corrupted. The most likely cause for mail file corruption is anti-virus software, particularly when accumulating messages in Inbox. You can attempt to repair the Inbox mail files. Note, it's quite possible that the damage has already be done, and your missing messages are gone for good. In that case you'd need to restore them from a recent back created prior to the incident. Try to rebuild the index file of Inbox. Right-click the Inbox folder in the folder pane - Properties - Repair Folder These are some generic suggestions to avoid problems with anti-virus software. Create an exception in your anti-virus software for the Thunderbird profile folder, so that the anti-virus real-time scanner will not scan it. https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data#w_how-to-find-your-profile Don't let your anti-virus software scan incoming and outgoing messages. Don't let your anti-virus software scan attachments. Don't let your anti-virus software intercept your secure connection to the server. Remove any add-ons your anti-virus software may have installed in Thunderbird. Keep it working. http://kb.mozillazine.org/Keep_it_working_-_Thunderbird And last but not least, backup your Thunderbird profile on a regular basis. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile