mail box rule
n. in contract law, making a written offer or acceptance of offer valid if sent in the mail, with postage, within the time in which the offer must be accepted, unless the offer requires acceptance by personal delivery on or before the specified date. The rule may also apply to mailing payments of insurance premiums when due. However, relying on this so-called "rule" can be dangerous, since the party awaiting the acceptance or payment may cancel the offer if there is no response in hand when the time runs out.