What Does the Mailbox Rule Provide

What does the mailbox rule provide?

A) that an acceptance must be mailed in order to be effective
B) that most contracting must be conducted through the mail
C) that an acceptance is not effective until it reaches the mailbox of the offeror
D) that a properly dispatched acceptance is effective when sent, even if it does not reach its destination
E) that all communications in connection with contract formation are effective when sent, so long as they are properly dispatched