I have set message retention rules for each folder under my Comcast mail account but they are not being followed. For the main Inbox folder I only keep messages for 15 days but most for most other folders in this account I have selected not to delete messages automatically and not to use the account setting. Thunderbird has deleted messages in these folders anyway. I suspect the Inbox retention rule is being invoked.

I have changed the Account settings to "Do not delete". Perhaps that will help.