My landlady is charging me a late fee for a check that was delivered to a wrong address, any suggestions on how to contest this?

I sent my rent to my landlady for this month on May 2nd. It was postmarked on May 3rd, but did not end up reaching my landlady (she asserts) until May 11th. The envelope was apparently put in the wrong mailbox, an error on the postal service, which further delayed the check. Had it been delivered correctly it would have made it to her by the 5th or 6th, which is within the grace period time frame. She is now charging me a 6% (of my rent) late fee, asserting that it was late. She does not care whose fault it was and insists that I pay. She says if I do not pay, she will take it out of my security deposit. Do I have any legal recourse to contest this?