Final answer:
The mailbox rule states that an acceptance of an offer is effective upon dispatch, not receipt, hence if acceptance is mailed before a rejection is considered, the acceptance is valid and a contract is formed.
Step-by-step explanation:
The statement is true. Under the mailbox rule, also known as the posting rule, an acceptance of an offer becomes effective when the offeree dispatches it, not when it is received by the offeror. Therefore, if the offeree sends an acceptance via mail and then decides to reject the offer afterwards, the original acceptance is still valid, and a contract is created at the time the acceptance was dispatched. This rule applies to minimize the risks that the offeree faces due to possible delays in communication.
It's important to note that this rule is somewhat modified by modern means of communication, and there are exceptions where the rule does not apply. However, in the context of this question and traditional mail communication, the mailbox rule is pertinent.