Contracts I
Restatement Second of Contracts § 40
Time When Rejection or Counter-Offer Terminates the Power of Acceptance
Rejection or counter-offer by mail or telegram does not terminate the power of acceptance until received by the offeror, but limits the power so that a letter or telegram of acceptance started after the sending of an otherwise effective rejection or counter-offer is only a counter-offer unless the acceptance is received by the offeror before he receives the rejection or counter-offer.
Copyright, The American Law Institute