The mailbox law, which is also known as the mailbox rule or posting rule is a legal principle contract law.
The mailbox law or legal principle contract law is a law that determines when an offer is considered to be accepted when communication is not instantaneous.
Most specifically under the mailbox law or mailbox rule, an acceptance of an offer is effective when the offer is properly dispatched, which means placed in the mail or otherwise sent for delivery.
This is true even if the offeror has not yet received it.
The mailbox law or mailbox rule is the default rule which determines when a contract has been formed when parties don't communicate with each other instantaneously.
An acceptance is considered to be effective as soon as it's properly dispatched, which means that it's placed in the mail with the correct postage and address.
The mailbox rule or mailbox law mainly applies to acceptances and not rejections or counteroffers, which are often effective only upon receipt.
The mailbox law or mailbox rule can also apply to acceptances that are sent via email, mail, fax or courier.
The mailbox law or mailbox rule is relevant when there is a time lag between sending and receiving a communication.
And the focus of the mailbox law and mailbox rule is on the offeree's action of sending of the acceptance, not on whether the offeroro has received it.