To make Mozilla Thunderbird delete old messages in a folder automatically:
* Click on the desired folder with the right mouse button.
* Select Properties... from the menu.
* Go to the Retention Policy tab.
* Make sure Use server defaults is not checked.
* Check either Delete all but the last __ messages or Delete messages more than __ days old.
* Enter the desired time or message count.
o Keeping about 30 days or 900 messages in a Trash folder usually works fine.
* Click OK.