From MozillaZine Knowledge Base
Beginning with version 1.5, Thunderbird has a new message aging feature that can be used to automatically delete messages from folders based upon the age of messages or the number of messages in a given folder. The message's sent date will be used to determine the age of a message, not the date when it was received. This feature can be used with e-mail, newsgroup, and RSS accounts.
Be warned: messages deleted via this feature will be deleted permanently, not sent to the Trash folder. It is strongly recommended that you experiment with the message aging settings using non-critical messages (such as in an RSS account) before using the message aging feature with messages that you would not want to risk losing.
The automatic deletion of messages will take place when you open a folder. If you change your settings (as described below) with a folder already open, the changes will not be reflected in the currently open folder until you select another folder and then go back to the first folder.
Warning: The checkbox "Always delete read messages" does exactly what it says. Thus, even if you have "Don't delete any messages" selected in the main option, checking this box will delete a message once it is read (this box will be hidden by default in Thunderbird 3.0).
Message aging settings must be made separately for each mail or newsgroup account; RSS accounts use only folder-specific settings. To activate message aging and set your preferences for a single account, go to:
You can also make folder-specific settings for message aging. For example, if the account is set to the default "Don't delete any messages", you could choose to have a single folder set to delete messages older than 90 days old. You can access folder-specific settings in either of these ways:
The label "Use server defaults" in the Folder Properties is misleading. Indeed, it does not refer to any setting on your provider's side, but simply means that your respective choices in the settings for the account are used where this folder is located.
Note: The folder-specific settings are kept in the ".msf" files, thus deleting those resets the retention policy to "use server defaults".