Follow these instructions to leave a copy of your mail on the server using Thunderbird:

1. On the Tools menu, click the Account Settings option.

2. Click the account you want to edit.

3. Select the Outgoing Server (SMTP) option, then select the SMTP server you wish to edit.

4. On the Server Settings page, select the Leave Messages on Server option.

Your mail now leaves a copy of the message on the server.