Mailbox Rule

The mailbox rule (also called the deposited acceptance rule) is a timing rule that determines when an acceptance becomes effective. Under the rule, an acceptance is effective the moment it is dispatched — that is, when it is placed out of the offeree's possession, such as being dropped in the mailbox. All other communications — offers, rejections, revocations, and counteroffers — are effective upon receipt.

The rule creates important timing implications. If an offeror mails a revocation and the offeree mails an acceptance before receiving the revocation, the acceptance is effective upon dispatch and the revocation arrives too late. Conversely, if the offeree mails a rejection and then changes their mind and mails an acceptance, the outcome depends on which communication arrives first — but most courts hold that the rejection controls if received first, preventing the offeree from speculating at the offeror's expense.

Several limitations apply. The offeror can opt out of the mailbox rule by specifying that acceptance is effective only upon receipt (e.g., "this offer is accepted only when I receive your written acceptance"). The rule applies only when the acceptance is sent via an authorized medium — generally, the same medium used by the offeror or one customary in the trade. If the offeree uses an unauthorized medium, acceptance is effective upon receipt rather than dispatch.

The rule does not apply to option contracts, where acceptance must be received before the option expires. Nor does it apply to electronic communications, where transmission and receipt are essentially simultaneous.

On contracts exams, the mailbox rule creates timing puzzles involving crossed communications. Students should create a timeline of dispatches and receipts to determine when each communication becomes effective.