Mailbox Rule

The mailbox rule, also called the posting rule or the deposited acceptance rule, is an exception in the contract law of common law countries to the general principle that acceptance of an offer takes effect only when it is communicated to the offeror. Under the rule, acceptance sent by post becomes effective the moment it is placed in the mail, rather than when the offeror actually receives it, while an offer itself and any revocation of an offer still take effect only on receipt, so a revocation must reach the offeree before that offeree posts an acceptance. The rule developed through nineteenth-century English cases beginning with Adams v Lindsell in 1818 and continuing through Dunlop v Higgins in 1848, Household Fire and Carriage Accident Insurance Co Ltd v Grant in 1879, and Henthorn v Fraser in 1892. Its underlying rationale is that an offeror who invites acceptance by post implicitly authorizes the post office to receive that acceptance on behalf of the offeror, which places the risk of a lost or delayed letter on the offeror unless the offer expressly requires actual receipt instead.