How does the postal rule apply to accepting and terminating contracts?

For a contract to be valid there needs to be both offer and acceptance. Therefore, a contract cannot be enforced unless it is accepted.
The postal rule states that an offer is deemed to be accepted from the time the acceptance was posted, presuming that a response via the post was in reasonable contemplation of the parties. This means that if A sends an acceptance of B's offer in the post on Tuesday, but B never receives the acceptance because it gets lost in the post, or he does not check his letter box, then B's offer was still accepted by A on Tuesday. There is no requirement for that letter to have ever actually have been received by B.
However, if B wishes to terminate the contract, and does so by post, this termination will not be valid until it is actually received by A.
In sum, to accept an offer A simply needs to correctly post the acceptance. To terminate a contract the termination actually has to be received.