mailing vs transmittal

mailing and transmittal both are nouns.

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

As nouns, transmittal is a hypernym of mailing; that is, transmittal is a word with a broader meaning than mailing:

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

Other hypernyms of mailing include transmission, transmitting.

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