Thousands of old emails marked as unread

Hello,

I have been using Windows Live Mail 2012 as my primary email app for a while now, with no trouble. But this afternoon, my account started downloading thousands of old emails from the past several months, which had already been read, sorted, and/or deleted, as if they are unread. Is there any solution to this problem? I would prefer to not have to switch to another email app.

I am using Windows 8.1.

Thanks!