This is known as a partial prepayment. The extra money may be applied assuming your loan is current and has standard application requirements; please check your Promissory Note / Mortgage and any applicable Riders for details.
Any amount, over and above the regular payment amount, will be applied first to any outstanding charges (late fees or interest) and then to principal.
Additionally, if your Note has a prepayment fee, the extra monies paid could trigger a penalty. Please examine your Note carefully to determine if a prepayment penalty applies before sending extra money.