Indian Post Office Act, 1898
5. Certain persons expressly forbidden to convey letters
Wherever within 5[India] posts or postal communications are established by the Central Government, the following persons are expressly forbidden to collect, carry, tender or deliver letters, or to receive letters for the purpose of carrying or delivering them, although they obtain no hire, reward or other profit or advantage for so doing, that is to say :-
(a) common carriers of passengers or goods, and their servants or agents, except as regards letters solely concerning goods in their carts or carriages; and
(b) owners and masters of vessels sailing or passing on any river or canal in 5[India], or between any ports or places in 5[India], and their servants or agents, except as regards letters solely concerning goods on board, and except as regards postal articles received for conveyance under Chapter VIII.