Erase old mail messages

I have all my mail accounts set up to erase deleted messages when one month old. Each one of my email accounts is a POP account that I have configured to remove the messages from the server after Mail downloads them. I also have each account configured to erase deleted messages when one month old, as indicated above. However, when I look at the messages in the trash folder, there are hundreds with a date much older than 30 days from today. Does anyone know why these would not have been erased?