Mailbox rule

The posting rule (or mailbox rule in the United States, also known as the "postal rule" or "deposited acceptance rule") is an exception to the general rule of contract law in common law countries that acceptance of an offer takes place when communicated. Under the posting rule, that acceptance takes effect when a letter is posted (that is, dropped in a post box or handed to a postal worker). In plain English, the "meeting of the minds" necessary to contract formation occurs at the exact moment word of acceptance is sent via post by the person accepting it, rather than when that acceptance is received by the person who offered the contract.

The rules of contracts by post (postal rules) include the following:

One rationale given for the rule is that the offeror nominates the post office as his or her implied agent, and thus receipt of the acceptance by the post office is regarded as receipt by the offeror. The main effect of the posting rule is that the risk of acceptance being delivered late or lost in the post is placed upon the offeror. If the offeror is reluctant to accept this risk, he can always expressly require actual receipt as a condition before being legally bound by his offer.

The rule was established by a series of 19th century cases, starting with Adams v Lindsell (1818) B & Ald 681, which was later confirmed and expanded in Dunlop v Higgins (1848) 1 HL Cas 381, Household Fire Insurance Company v Grant (1879) 4 Ex D 216 and Henthorn v Fraser [1892] 2 Ch 27.

The posting rule applies only to acceptance. Other contractual letters (such as one revoking the offer) do not take effect until the letter is delivered, as in Stevenson, Jacques & Co v McLean (1880) 5 QBD 346. The implication of this is that it is possible for a letter of acceptance to be posted after a letter of revocation of the offer has been posted but before it is delivered, and acceptance will be complete at the time that the letter of acceptance was posted—the offeror's revocation would be inoperative.

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