Hello jlnunny The first thing to do is to try to figure out what the missing messages have in common. For example, is the problem limited to one specific folder or account or just new messages? Did you lose the messages shortly after changing the configuration? Look for anything unusual or that doesn't look right (besides some of your messages being missing). Try the below solution 1. Make a new folder in your account; 'SentOld'. 2. Move all your sent emails from 'Sent' into this folder. 3. Close Thunderbird 4. Navigated to your Mail folder and deleted 'Sent.msf' and 'Sent' 5. Open Thunderbird and allow it to reinstall 'Sent' and a 'Sent.msf' folders. 6. Check that a sent message now appears in the 'Sent' folder.