Do you have any filters operating? EG: show only unread mail ? This can occur if you have selected the icon to 'Show only unread messages' on Quick Filter Bar. Also check to see if the Quick Filter Bar 'pin' icon has been selected so using a filter preset on another folder. Check to see if you have this selected : * View > Threads > All Have you accidentally selected to hide those emails eg: mark as ignored ? This can easily be done by shortcut keys. Maybe you have accidentally pressed the 'K' key which auto ignores aka 'hides' a thread or email ? * https://support.mozilla.org/en-US/kb/keyboard-shortcuts-thunderbird * https://support.mozilla.org/en-US/kb/ignore-threads You could also select to display ignored threads meaning you can then see if a thread is marked as ignored. View > Threads > Select 'Ignored Threads' Ignored Threads will have an icon in the Threads column header list- a red circle with red line across center. Right click on email and the 'Ignored Thread' option is selected - click on it to remove the setting.