Receiving a lot of emails takes up disk space, and if your old emails are not periodically deleted, you will not be able to receive new emails. The following is a step-by-step guide to automatically deleting emails from the server in Mozilla Thunderbird 4.0 and above.
1. In Mozilla Thunderbird, choose Tools and then Account Settings.
2. Click Server Settings listed below the account you want to edit.
3. Select the Leave messages on server option.
4. Select the For at most option.
5. Choose the amount of time you wish emails to stay in your account. I suggest choosing 7 (or lower) to avoid any problems.
6. Click OK.