The postal rule is an exception to the general rule that contract acceptance must be communicated directly to the offeror and acceptance is only effective when the offeror receives that acceptance. The acceptance is actually effective as soon as the offeree mails the acceptance, according to Cornell University Law School.
The acceptance must be mailed in a timely manner or according to the terms of the offer. The parties to the contract can alter the terms of the offer so the postal rule, or mailbox rule, is ineffective. It does not matter when the offeror actually receives the acceptance for it to be effective, according to National Paralegal College.