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:

1.  An offer made by post/letter is not effective until received by the offeree.

2.  Acceptance is effective as soon as it is posted.

3.  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.