They claim they didn't receive our check by due date

Last year our mortgage was sold to America's Servicing Company. Our first payment in 2007 was due on January 22. We sent the payment 5 business days in advance (we always do so). However, America's Servicing Company claims that they didn't receive the check by the due date, and hit us with additional fees because of "the late payment".

I've been shouted at and treated with absolute disregard. Every representative was rude to me, and didn't want to take the time and look into the problem. It's obvious that this delay was not my fault - I have proof from my bank that the payment was sent out well before the due date. However, this all doesn't matter, they just want my money.