This brief 2-step guide will help you to set Thunderbird to leave mail on the server. Leaving your mail on the server can be helpful if you use Thunderbird on a portable USB drive to check your email while away from your main computer, or if you have multiple computer/email clients checking email for the same account (for example MacMail on one laptop, and Thunderbird on another).
This way when you return to your main desktop/laptop, you can still download all of your email. These steps can be applied to most other email clients, although the menu layout may be slightly different for each email client, the general procedure is very similar. Note that this just applies to POP accounts – if you have set your email program only to use IMAP, then messages stay on the server until you delete them.
1. Launch Thunderbird and select Tools from the top menu, then Account Settings… from the drop-down list. (Tools > Account Settings)
2. Select Server Settings from the left window. In the Server Settings section of the right window, place a check in the box labeled “Leave messages on server”. Then you can click OK, OR also set Thunderbird to leave the messages on the server for a set number of days, simply tick the box and input the number of days you would like to keep the mail on the server for.