transmittal vs mailing

transmittal and mailing both are nouns.

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

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

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

Other hyponyms of transmittal include forwarding, posting, telephotography.

transmittal (noun)	mailing (noun)
the act of sending a message; causing a message to be transmitted	the transmission of a letter
	mail sent by a sender at one time