Answer:
The mail was delivered by them is the passive voice.
Step-by-step explanation:
The Passive voice is used when we want to emphasize the verb i.e the action and the object of a sentence rather than subject. in other words, The passive voice is used to indicate the object that experiences an action rather than the person or the object that performs that action.
Therefore, the passive voice of 'when did they deliver the mail' is The mail was delivered by them. showing that the object is acted upon by the verb. The active voice is usually more direct than the passive voice. In The passive voice, the subject is either less important or that it is not known.