transmittal vs sending

transmittal and sending both are nouns.

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

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

transmittal: the act of sending a message; causing a message to be transmitted
sending: the act of causing something to go (especially messages)

transmittal (noun)	sending (noun)
the act of sending a message; causing a message to be transmitted	the act of causing something to go (especially messages)