The mailbox rule, also called the posting rule, refers to the default rule in contracts law for determining when an offer was accepted. Under the mailbox rule, an offer is considered accepted the moment the offeree mails their letter, rather than when the offeror receives the letter in the mail. The mailbox rule also applies to other means of communication, such as a fax, telegram, or email, provided that it is irrevocable once sent.
The mailbox rule originated in the British case of Adams v. Lindsell (1818). In that case, the Court determined that the buyer accepted the seller’s offer to buy wool when they responded to the seller’s letter, not when the seller received their reply.
As a default rule, it does not apply in all cases. Parties can contract around the mailbox rule to change at what time an offer will be considered accepted.
As with most of contract law, the mailbox rule varies from state to state. In a minority of states, such as California, the mailbox rule applies to option contracts. In most states, however, it applies only to bilateral contracts.
Restatements
Restatement (Second) of Contracts § 63 (1981) echoes the opinion of many states, distinguishing between an option contract and a bilateral contract. The Restatement reads: "Unless the offer provides otherwise, (a) an acceptance made in a manner and by a medium invited by an offer is operative and completes the manifestation of mutual assent as soon as put out of the offeree's possession, without regard to whether it ever reaches the offeror; but (b) an acceptance under an option contract is not operative until received by the offeror."
In other words, under jurisdictions which have adopted the Restatement rule, the mailbox rule doctrine applies to bilateral contracts, but not to option contracts.
[Last updated in July of 2023 by the Wex Definitions Team]