My email has been set up on POP 3 for years, placing a copy on server so I can check email from multiple computers. I understand this isn't a good way to do email, and that converting to IMAP would be wiser.
I created a new IMAP account in TB, but kept the POP3 active since I've got a large archive I don't want to lose.
Typically, I receive a message from a customer and keep it in my inbox until their order is complete or issue is resolved, then I delete the message and it goes into the trash folder, which is my archive.
I had to do a Windows update that required a restart, and when I launched TB, all the read messages that were in my IMAP inbox were gone! Fortunately, the POP3 still had the messages. None of my settings in TB allow for emptying of the inbox on exit. I assume an IMAP account would have to leave the messages on the inbox to allow for other computers to access them, so something's not right.
OK, I think I may have figured it out - I changed my POP3 to not leave a copy on the server, thinking the IMAP would do that. But I should have disabled the POP3 from downloading messages too!
If only the IMAP (not the POP3) downloads a message, it won't be deleted until I delete it.
We'll see if things behave...