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A mailed contract is accepted when the letter is posted

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); the post office will be the universal service provider, such as the UK's Royal Mail, the Australia Post, or the United States Postal Service. 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:
An offer made by post/letter is not effective until received by the offeree. Acceptance is effective as soon as it is posted. For revocation to be effective, it must be received by the offeree before they post their letter of acceptance. 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 Court of Appeals for the Third District of Texas considered the mailbox rule in 1994 in the case of Cantu v Central Education Agency and others. The Texas Business and Commercial Code, following the Uniform Commercial Code, states that "unless otherwise unambiguously indicated by the language or circumstances, (1) an offer to make a contract shall be construed as inviting acceptance in any manner and by any medium reasonable in the circumstances". Cantu had hand delivered a letter resigning from an offered contract of employment to the Central Education Agency's office in San Benito on a Saturday, and it was therefore read by its recipient on the Monday morning. The letter stated that outstanding pay should be forwarded to an address in McAllen, around from San Benito. A letter was posted back on the Monday evening, accepting the resignation. On the Tuesday morning, Cantu hand delivered a letter withdrawing her resignation, but the superintendent of schools advised her that her resignation had already been accepted, by virtue of an acceptance letter being posted to the McAllen address. Cantu argued that postal acceptance was not valid because her withdrawal letter had been hand delivered. The trial court and the appeal court accepted the Agency's opinion that it was reasonable, taking account of all the circumstances, for the Agency to accept the resignation by mail. The majority rule in the United States is that the mailbox rule does not apply to option contracts. By default, an option contract is accepted when the offeror receives the acceptance, not when the offeree mails it. However, because the California Civil Code applies the mailbox rule to all contracts, California follows the minority rule, under which the mailbox rule does also apply to option contracts.

In Tallerman & Co Pty Ltd v Nathan's Merchandise (1957) 98 CLR 93, 111 112 Dixon CJ and Fullagar J took a more restrictive view. "The general rule is that a contract is not completed until acceptance of an offer is actually communicated to the offeror, and a finding that a contract is completed by the posting of a letter of acceptance cannot be justified unless it is to be inferred that the offeror contemplated and intended that his offer might be accepted by the doing of that act." The High Court included the element of intention.

Civil law jurisdictions do not follow the postal rule. The classical civil law position is that acceptance, like any expression of will, can only be effective if it was communicated to the addressee, unless the lack of communication can be attributed to the latter. The Vienna Convention on the International Sale of Goods chooses a compromise between the two approaches: According to article 18(2) of the Convention, an acceptance is effective when it reaches the offeror. However, article 16(1) of the Convention provides for the most important consequence of the common law "posting rule", that is, an offer may not be revoked if the revocation reaches the offeree after it has dispatched an acceptance.