The use of the postal service is customary, so acceptances are considered effective when mailed, regardless of the method used to transmit the offer. Indeed, the so-called mailbox rule has a lineage tracing back more than one hundred years to the English courts. 1
The mailbox rule may seem to create particular difficulties for people in business, since the acceptance is effective even though the offeror is unaware of the acceptance, and even if the letter is lost and never arrives. But the solution is the same as the rationale for the rule. In contracts negotiated through correspondence, there will always be a burden on one of the parties. If the rule were that the acceptance is not effective until received by the offeror, then the offeree would be on tenterhooks, rather than the other way around, as is the case with the present rule. As between the two, it seems fairer to place the burden on the offeror, since he or she alone has the power to fix the moment of effectiveness. All the offeror need do is specify in the offer that acceptance is not effective until received.
In all other cases—that is, when the offeror fails to specify the mode of acceptance and the offeree uses a mode that is not reasonable—acceptance is deemed effective only when received.