What to do with an invalid default notice?

We received a default letter from our mortgage company for $500. We've never missed a mortgage payment and have the bank statements to prove it. We also have the mortgage company's statements that don't mention anything about us needing to pay an additional $500. We also have an automated recording from them that says we are current on all our payments. We are planning on just paying the $500 because the letter is threatening and honestly, quite terrifying. No one wants to be told they will lose their house if they don't pay $500. My husband and I both have credit scores of 820+, we've never missed a payment, and we have multiple investment properties that we've never missed payments on. A mistake like this is incredibly threatening and distressing. Not only does this affect our personal lives, but also our entire business and livelihood. Is this a valid mistake or can we take any legal action against them? I really appreciate any help with this. Thank you-