Changing the Mozilla Thunderbird settings
To leave messages on the server, you will:
1. Start the Mozilla Thunderbird program.
When Thunderbird opens, click Tools, then Account Settings from the menu bar.
2. The Account Settings window will appear.
Click the small "plus sign" to the left of the account to expand the list beneath it.
Click Server Settings.
Check the box next to Leave messages on server. Untick the Until I delete or move them from Inbox and For at most X days.
3. Close the Account Settings window.