In Thunderbird version 17.0.8 running on WinXP, I setup my Verizon mail account as a POP mail account, pulling messages from my Verizon.net mailbox
I have 2+ years worth of messages and am hitting the mail quota. I want TB to pull all of my messages from the server, but not delete them for 90 days so that I have time to sync them to my other devices. I set the Server Settings by checking "Leave messages on server" and "For at most 90 days", like so:

So I would expect anything older than 90 days should get deleted from the server after TB pulls it, correct? Nothing seems to get deleted, no matter how many times I fetch mail. I tried changing that 90 days to different values, and even tried recreating the Account - no luck.
How do I get TB to remove old messages from the server automatically based on date?
Thanks!