c. 1400, from Latin transmittere "send across, cause to go across, transfer, pass on," from trans "across, beyond" (see trans-) + mittere "to release, let go; send, throw" (see mission). Related: Transmitted; transmitting.
suffix forming nouns of action from verbs, mostly from Latin and French, meaning "act of ______ing" (such as survival, referral), Middle English -aille, from French feminine singular -aille, from Latin -alia, neuter plural of adjective suffix -alis, also used in English as a noun suffix. Nativized in English and used with Germanic verbs (as in bestowal, betrothal).
<a href="https://www.etymonline.com/word/transmittal">Etymology of transmittal by etymonline</a>
Harper, D. (n.d.). Etymology of transmittal. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/transmittal