transmission vs mailing

transmission and mailing both are nouns.

Word	Noun	Adjective	Verb	Adverb
transmission	Yes	No	No	No
mailing	Yes	No	No	No

As nouns, mailing is a hyponym of transmission; that is, mailing is a word with a more specific, narrower meaning than transmission:

transmission: the act of sending a message; causing a message to be transmitted
mailing: the transmission of a letter

Other hyponyms of transmission include forwarding, posting, telephotography.

transmission (noun)	mailing (noun)
the act of sending a message; causing a message to be transmitted	the transmission of a letter
the gears that transmit power from an automobile engine via the driveshaft to the live axle	mail sent by a sender at one time
communication by means of transmitted signals
an incident in which an infectious disease is transmitted
the fraction of radiant energy that passes through a substance