How are they getting deleted? Are you deleting them from you computer or phone?

Thanks for following up. I am deleting the messages in thunderbird on the computer. There are no other devices associated with the account.

First it would help to understand how IMAP works. It is server based email. Messages are on the server and you remotely view them on your devices. It you delete a message from a device or directly at the server the message is gone. If you want the message to stay on the server, stop deleting them. If you are trying to unclutter the Inbox then create a new folder for those messages that you want to keep and move the messages there. The new folder needs to be on the server and not in Thunderbird Local Folders to be on the server. Local Folders are just that. They are on your computer.

That's how IMAP works, Stop deleting them.

Thanks Airmail. That has helped make it clearer.

However, when I set up outlook.com to retrieve messages, it managed to retrieve even the ones that seemed to have been deleted and removed from the server (by thunderbird's IMAP). So what I would like to know is where these were being stored and how to access them.

My guess is that you also do not know anything about the Compact function in Thunderbird. When you delete something in Thunderbird it marks it for deletion and hides it from view. Compacting actually removes the messages and frees the space on the drive. So your mystery messages were just marked for deletion and Outlook found them. I do not care what email client you use. IMAP works the same.

Thank you!