Restatement Second of Contracts § 63
Time When Acceptance Takes Effect
Unless the offer provides otherwise,
- an acceptance made in a manner and by a medium invited by an offer is operative and completes the manifestation of mutual assent as soon as put out of the offeree's possession, without regard to whether it ever reaches the offeror; but
- an acceptance under an option contract is not operative until received by the offeror.
Copyright, The American Law Institute