IMAP mail accounts see a remote view of the server. You subscribe to see server Folders. Headers are downloaded and when you select to view an email, the emails are downloaded to a temp cache to enable quicker access to those emails, but upon exit Thunderbird the temp cache is lost, so you cannot read emails if offline. These emails are not stored on your computer. If you synchronise subscribed folders, then a copy of the folder is downloaded to Thunderbird Profile folder and stored in an mbox file. This means you can read them even if offline. However, the folder is set up to synchronise with the server when changes occur, so that your copy and the folder on the server are updated to be the same. I would recomend that you do not allow folders to get too big as it would take longer to synch. more info on IMAP: https://support.mozilla.org/en-US/kb/imap-synchronization Can you see an 'All Mail' folder? This is gmails copy of all your emails, so it is gmail Archive of your emails. This folder can get huge. As it only shows you what you already have in other folders, it also doubles the size of your Profile and can take a while to download. It is recommended that you do not subscribe to see this folder. Read info at link - under 'all Mail' section: http://kb.mozillazine.org/Gmail