When an offer does not specify a means of acceptance, it can be accepted by any means reasonable under the circumstances.

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This is known as the "mailbox rule" and is a common law principle. As long as the means of acceptance is reasonable under the circumstances, it is considered effective upon dispatch (i.e. when it is sent), even if the offeror does not receive it immediately. However, if the offer specifies a particular means of acceptance (e.g. email only), then that method must be used for the acceptance to be effective.

When an Offer Does Not Specify a Means of Acceptance