A “Man in the middle attack,” depends on controlling the exchange between the sender and receiver. The principle is to intercept the “sent” message. Read it (and or modify it,) then pass it on to the receiver, without making him or her aware of the substitution.
This procedure is open to various failings. One possibility is for the receiver to discover discrepancies in the message during face to face review with the sender. This might be the gold standard, since it is impossible to counterfeit.
However, if the sender and receiver creatively record the time, it would become possible to observe that the message lingered too long in transit. This might be more practical for couriers, than for computer bits, because of the vanishingly small time frames. However, atomic clocks delineate time very precisely.