Obviously after formatting the computer everything on it is completely wiped. IMAP mail accounts are designed to give you remote access to the server. Emails are auto stored on the server because you may use various devices to access the account. You have set up an IMAP mail account which allows you to subscribe to see any folder and it's contents kept on the server. You would see the same folders and emails if you logged on to your webmail account via a browser. If you had previously used a POP mail account, then those emails would be downloaded to your Thunderbird profile and usually they are deleted off the server. This is normal default practise. It also prevents you from using up your limited server quota. You would then make regular backups of downloaded emails/profile folder and store them on an external device. However, there is an option in Thunderbird Account Settings > Server Settings to select 'leave messages on the server'. But you would have to select this after creating the account before your initial download if you wanted to keep all copies on server. Did you previously use a pop mail account? If yes, did you backup your profile / emails ?