Do you use Imap mail accounts ? If yes, then emails are stored on the server and each imap account hooks up to server to display it's contents. This means any moving or deleting etc that you do in imap account on desktop gets imap folders sycnhronised to server folder, so when you then fire up Thunderbird on laptop, that imap account will look at same server folders and synchronise them to be identical to server. Imap accounts were designed to faciliate access from various computers and still see all the same emails. Create same imap account on laptop. Please note: Do not assume any imap folder is like a backup copy. As imap synchronise with server, they cannot be regarded as independent from server. However, you can make sure all folders are set up to receive full copies of downloaded mail and then create a backup - ideally use Thunderbird to access profile folders, exit Thunderbird and then copy the 'Appdata/Roaming/'Thunderbird' folder to an external device.