[Dovecot] Dovecot 1.2.16 and Thunderbird 5 - pop3 mails not getting deleted

I run a Dovecot 1.2.16 pop3 server and have just started using the Thunderbird 5 email client.

Thunderbird is set to leave the messages on the server unless they are over 91 days old. However, I'm not seeing it delete any of the older messages.

I'll run a debug run of Thunderbird when I get back home, but I'm just wondering if there's any gotcha's I should be aware of with Thunderbird and Dovecot in a pop3 environment. The Thunderbird folks are wondering if it's the server not the client.

Thanks.