I received a letter saying I was overpaying my mortgage by $700/month and got an $8,000 refund check — is this normal?

A refund like this can be completely normal, but an $8,000 check is worth verifying before you spend a dollar of it. If your loan has an escrow account for property taxes and insurance, your servicer has to analyze that account at least once every twelve months. When the analysis shows the account holding a bigger cushion than the law allows, the servicer must send the excess back to you. That is very likely what your check is. The size is what deserves a closer look. A $700-a-month overage on taxes and insurance is unusually large for most of the country, so read the escrow analysis that came with the check and trace where the gap came from. We have seen a servicer misread county assessor data and set the escrow figure wrong, and an error in either direction eventually corrects itself in a way you would rather see coming. One practical caution: if the account was overfunded because your taxes or insurance were estimated too high, next year's analysis can swing the other way, and your monthly payment can rise to rebuild the required cushion. Confirm the underlying tax and insurance numbers before treating the surplus as permanent. If the figures will not reconcile, call the servicer and have them walk you through the analysis line by line.