• Menu app icon > Help > Troubleshooting Information

Under Application Basics section Left Pane - About half way down is 'Profile Folder', adjacent in right pane is 'Open Folder'

• click on 'Open Folder' button.

a new window opens showing the contents of your 'profile name' folder.

• Exit Thunderbird now - this is important

• Scroll down to locate 'virtualFolders.dat'
• delete: virtualFolders.dat file

• click on 'Mail' folder to see contents.
• delete 'smart mailboxes' folder.

• Start Thunderbird.

did not work for me.

This worked for me!

N4NW You also posted in another question. You said "I still am missing messages that were in the original 3 mail accounts inboxes before running the unify instructions to create a global inbox. Where did these messages go?" So you do not have the same issue. You do not see the emails in the Unified Inbox because they are not in the accounts Inbox. So there is nothing wrong with your Unified Inbox. You mention unify instructions but there is a difference between a Global inbox and a Unified Inbox. The 'Chosen Solution' will not work for you because you do not have the same issue as the Question.

This did not work for me either. After deleting the smart mailboxes a couple of times, I realized that at first it worked and then after a second start up, it stopped working again.

After just deleting the folder and opening Thunderbird, the language of the mailboxes was English, so the "sent" folder (the one I had problems with) was named "sent" and it worked. After a second start up the folder changed its name to Spanish (my language in TB). So clearly the problem was in the mailboxes created and how the translations were not working well.

I edited the file sent.msf located inside the smart mailboxes and edited the file to replace "enviados" with "sent" and now it is so far working.