If you use a POP mail account then all mail is downloaded from server and stored in your Thunderbird Profile folder. If you use IMAP, you see a remote view of folders by subscribing to see folders on server. Headers are downloaded and when you select to view email, it is retrieved from server and stored in a temp cache. The emails are not stored in the Thunderbird Profile folder and so cannot be read when offline. However, if you synchronise those subscribed folders then a copy is downloaded to Thunderbird Profile folder. See more info on this here: * https://support.mozilla.org/en-US/kb/imap-synchronization