UCC 3-603 provides this (take note of (b)):
§ 3-603. TENDER OF PAYMENT.
(a) If tender of payment of an obligation to pay an instrument is made to a person entitled to enforce the instrument, the effect of tender is governed by principles of law applicable to tender of payment under a simple contract.
(b) If tender of payment of an obligation to pay an instrument is made to a person entitled to enforce the instrument and the tender is refused, there is discharge, to the extent of the amount of the tender, of the obligation of an indorser or accommodation party having a right of recourse with respect to the obligation to which the tender relates.
(c) If tender of payment of an amount due on an instrument is made to a person entitled to enforce the instrument, the obligation of the obligor to pay interest after the due date on the amount tendered is discharged. If presentment is required with respect to an instrument and the obligor is able and ready to pay on the due date at every place of payment stated in the instrument, the obligor is deemed to have made tender of payment on the due date to the person entitled to enforce the instrument.
So naturally, you want to know why the creditor would be so stupid as to return your overdue payment. for the answer see (a) above, and paragraph two of section 1 of the uniform covenants of your mortgage or deed of trust:
“Payments are deemed received by Lender when received at the location designated in the Note or at such other location as may be designated by Lender in accordance with the notice provisions in Section 15. Lender may return any payment or partial payment if the payment or partial payments are insufficient to bring the Loan current. Lender may accept any payment or partial payment insufficient to bring the Loan current, without waiver of any rights hereunder or prejudice to its rights to refuse such payment or partial payments in the future, but Lender is not obligated to apply such payments at the time such payments are accepted. If each Periodic Payment is applied as of its scheduled due date, then Lender need not pay interest on unapplied funds. Lender may hold such unapplied funds until Borrower makes payment to bring the Loan current. If Borrower does not do so within a reasonable period of time, Lender shall either apply such funds or return them to Borrower. If not applied earlier, such funds will be applied to the outstanding principal balance under the Note immediately prior to foreclosure. No offset or claim which Borrower might have now or in the future against Lender shall relieve Borrower from making payments due under the Note and this Security Instrument or performing the covenants and agreements secured by this Security Instrument.”