You set up a new existing mail account for the new email address. If you are changing ISP, they will offer a new email address and it will use their server settings. If you currently use an IMAP mail account, then you need to synchronise your subscribed folders to get a copy in thunderbird. * https://support.mozilla.org/en-US/kb/imap-synchronization#w_configuring-synchronization-and-disk-space-usage Go into 'offline' mode and check you can still read emails. This will confirm you have a copy in thunderbird. Then still in offline mode, Right click on highlighted emails and use 'copy to' suitable folders in 'Local Folders' mail account. This will ensure you still have access to emails via Local Folders mail account after the imap mail account ceases to exist on server.