It all depends upon what type of mail account you are using. '''If you have a POP mail account:''' Thunderbird can only access the server Inbox. It downloads emails not previously downloaded to the mail account Inbox. These emails are stored in Thunderbird Profile on your computer. It is usual to remove the copy on the server. You can set it up to keep a copy on the server. However, you would need to be aware that you might reach your quota on the server. So it is best to perform backups of your Thunderbird Profile and choose a setting that only keeps a limited amount on the server eg: 60 days. The settings are here: Tools > Account Settings > Server Settings for the POP mail account OR Right click on mail account and choose 'Settings' to get to Account Settings. * Select: 'Leave messages on server' * Select: 'For at most eg: 60 days' * Select: 'Until I delete them' * Click on 'OK' ----------------------------------------------- '''If you have an IMAP mail account''' IMAP mail accounts see a remote view of the server folders. IMAP mail accounts can see any subscribed folder on the server. If you delete an email via Thunderbird or webmail using a browser, it will delete from both as they are one and the same folders. You can move/copy an email and save it in 'Local Folders' which is a special account in Thunderbird. These emails will be saved in Thunderbird Profile and are not effected by the server. However, you do need to backup these emails as you would have the only copy if removed from the server. Subscribed folders download headers and then download the rest of the email to a temp cache for easier retrieval, but they are not stored on your computer nor in the Profile folder. They are stored on the server. You cannot read these emails if you are in 'offline' mode. You can synchronise subscribed folders to get a copy downlaoded to your Thudnerbird Profile folder. These types of folders are constantly updating/synchronising with the server, so that both are the same. But you can read these emails even if using 'offline' mode. More info on Synchronising: * https://support.mozilla.org/en-US/kb/imap-synchronization ---------------------------------------------- Info on Backup: * http://kb.mozillazine.org/Profile_backup * http://kb.mozillazine.org/IMAP_backup