Maybe a problem with the index files. * Menu icon > Help > More Troubleshooting Information * Under 'Application Basics' - half way - Profile Folders - click on 'Open Folder' A new window opens showing the contents of your profile name folder. * Exit Thunderbird now - this is important If POP account: * click on 'Mail' folder * click on 'pop account name' folder Files that do NOT have any extension are mbox files and they contain emails - do not delete any of those mbox files. * Delete all the index files - they will have a *'''.msf '''extension. If you have subfolders then you will see folders with a *'''.sbd''' extension * Select each *.sbd folder in turn and delete all the files that have a *.msf extension. Start Thunderbird. All the index files will get recreated.