Email client downloading ALL emails, every time! WTF?

I use WinXP Home SP2, with Thunderbird as my email client. My email account is through my university, and is a regular old POP/SMTP set-up.

I have Thunderbird set to:

Leave messages on server.
Until I delete or move them from Inbox.

This seems to be the best set-up for me. It allows me to download all incoming emails to my computer, but also keeps a whole bunch of them on the webmail in case i need to access old emails when i’m away from my house.

Every week or so, i go into Thunderbird and move a whole bunch of emails from the Inbox to my archive folders. One they are moved, the Thunderbird settings ensure that they are also removed from the webmail. I leave about a 4-6 weeks worth of email in Thunderbird’s Inbox, giving me a 4-6 week collection of emails that remain on the server and can be accessed from the road.

Now, until last week, this system worked absolutely fine. Whenever i opened Thunderbird, any new emails would download to my computer, and would also be left on the server until i shifted them from the Inbox. Somehow (not sure exactly how it works) Thunderbird would only download new emails; it seemed to know which emails had previously been downloaded, and would leave those alone.

But all that changed last Friday. Suddenly, every time i opened Thunderbird and checked for new mail, it would begin to download all 600+ emails from the server—the new emails, plus all the ones that were already in my Thunderbird Inbox.

Of course, the first thing i did was check my account settings in Thunderbird to make sure that nothing had changed, but everything seemed just fine. I’ve sent an email to my university’s support center, asking if it might be something to do with their server, but they have so far been singularly unresponsive. The sum total of their replies so far has been an email saying:

Not sure why this is especially relevant, nor why eight question marks were necessary, but anyway.

Now, here’s another little tidbit of information that may or may not be relevant. A few weeks ago, our whole university mail system was shifted to new servers, and we all had to change our POP and SMTP addresses. This worked fine, and everything seemed great.

Then, last Friday, i received an email from the university’s email people welcoming me to the university email system, and directing me to the webpage for learning how to configure email clients. This email was not, in itself, remarkable in any way BUT it was the last email i received before the above-described problems began, so i’m a bit suspicious.

If anyone can hazard a guess as to what might be going on here, i’d be most grateful to hear it.