A more thorough explanation:

The mailbox rule is a legal principle that determines when an offer is accepted in a contract. It states that an offer is considered accepted at the time that the acceptance is communicated, whether by mail, email, or any other means of communication. This means that if you accept an offer by sending an email, the acceptance is considered valid as soon as you hit the send button, even if the email hasn't been received by the offeror yet.

For example, if you receive a job offer by email and you accept it by replying with an email, the acceptance is considered valid as soon as you hit the send button, even if the employer hasn't received the email yet. This means that the job offer is now legally binding, and you have accepted the terms of the contract.

However, parties can agree to not use the mailbox rule and determine between themselves at what time an offer will be considered accepted. For example, they can agree that an offer is only considered accepted when it is received by the offeror.

The mailbox rule originated in the British case of Adams v. Lindsell in 1818, and it has been adopted by many states in the US. However, the rule varies from state to state, and some states have different rules for option contracts.

Option contracts are contracts that give the holder the right, but not the obligation, to buy or sell an asset at a predetermined price. In some states, the mailbox rule only applies to bilateral contracts, which are contracts where both parties make promises to each other. However, in California, the mailbox rule also applies to option contracts.

For example, if you have an option contract to buy a house, and you exercise your option by sending a letter in the mail, the exercise of the option is considered valid as soon as you put the letter in the mailbox, even if the seller hasn't received the letter yet. This means that the seller is now legally obligated to sell you the house at the predetermined price.