The mailbox rule is a doctrine in contract law which states that if mail is a reasonable method of conveyance for an acceptance, the acceptance becomes valid on the date that it is dropped in the mailbox and postmarked. By extension, this rule is also used for payments on insurance premiums, in which the payment is considered a form of acceptance of the contract, and some tax authorities also consider the postmarked date to be the acceptance date. The mailbox rule was established under the rule of law in the 1800s, and is found in the legal standards of many nations.
There are a number of alternate names for the mailbox rule, including the postal acceptance rule, the posting rule, and the postal rule. It is important to note that this rule only holds true for acceptances. If someone mails a rejection of an offer, it is considered valid on the date it is received, not on the date that it is mailed.
In some cases, a contract will specifically address the mailbox rule by making it clear that a response must be received by a set date. In these instances, if someone waits until that date to mail an acceptance, it may not be accepted, because it will be received after the commitment date. Clauses of this type are sometimes used when people wish to avoid drawn out negotiations over a contract; if the deadline is based on when a response of any kind is received, it frees up the person who offers the contract to choose someone else if no response arrives in time.
The mailbox rule also comes into play when conflicting documents are sent. For example, if someone decides to reject an offer, and later to accept it, there is a question about which is valid. Under the mailbox rule, it is the acceptance which takes precedence, unless the rejection arrives before the acceptance is mailed. Likewise, if someone makes an offer and then revokes it, but the person has already sent a letter accepting it, the offer must be made good.
It has also been established in court that someone doesn't need to receive an acceptance for the acceptance to be valid, unless the sender of the acceptance has reason to think that it was not delivered. Once someone has dropped the acceptance in the mailbox, the deal is on, unless that person noticed that something happened at the post office which might have impeded delivery. For example, if the mailroom is flooded immediately after the letter is placed in the box, the acceptance would not be considered valid.