The Mailbox Rule in Contract Law

The mailbox rule states that an acceptance is effective when dispatched (put in the mail, sent electronically), not when received. All other communications — offers, rejections, revocations, and counteroffers — are effective when received. This means there can be a valid contract before the offeror even knows about the acceptance.

The mailbox rule was created in the 1818 English case Adams v. Lindsell to resolve a practical problem: if acceptance required receipt, each party would need to confirm receipt of the other's confirmation, creating an infinite loop. The rule picks a clear moment of formation — dispatch — and puts the risk of lost communications on the offeror (who can always specify “acceptance effective only upon receipt”).

Crossing communications: A mails acceptance on Monday. B mails revocation on Monday. Both are in transit Tuesday. Acceptance was dispatched before revocation was received → valid contract formed when A mailed the acceptance.

Rejection then acceptance: A mails rejection on Monday, then changes mind and mails acceptance on Tuesday. Whichever arrives first controls. If acceptance arrives first, there's a contract. If rejection arrives first, no contract (even though acceptance was dispatched).

Lost acceptance: A mails acceptance but it's lost in the mail. Under the mailbox rule, the contract was formed when the acceptance was dispatched — the lost letter doesn't undo the contract.

The mailbox rule does NOT apply when:

- The offeror specifies acceptance must be received (“acceptance effective upon receipt”)
- Option contracts (acceptance must be received before the option expires)
- UCC Article 2 follows the mailbox rule but with some modifications for merchants
- The acceptance is sent by an unreasonable method (e.g., acceptance by horse courier when offer was emailed)

In the age of email and instant messaging, the mailbox rule matters less for most communications (dispatch and receipt are nearly simultaneous). But it still appears on exams frequently because it tests your understanding of offer/acceptance timing and contract formation principles.