Definition - What does Mailbox Rule mean?
The mailbox rule is a rule that states that a contract offer is considered formally accepted when it is signed and mailed with appropriate postage. However, if there is an expiration date for the contract, then the signing party must mail the signed contract before the deadlines expires. Otherwise, it won't be valid.
Justipedia explains Mailbox Rule
The reason why the mailbox rule exists is to give structure to the timing of contracts. Whether or not a contract is valid can have major legal implications. So, it is important for anyone signing a contract to know that as soon as they mail a signed contract, that contract has officially become valid unless it was mailed after the deadline set by the offering party. If the signing party mails the contract before the deadline expires, then they become liable for upholding their contractual responsibilities.