We have a site in production that uses ECM 2.1 (before EXM). The production environment consists of a CM server and 2 CDs.

There is some code running on the CD servers that creates and sends messages using AyncSendingManager. Much the same as this post from briancaos:

https://briancaos.wordpress.com/2016/03/19/sitecore-exm-send-an-email-from-code/

Could there potentially be any way to make sure that the message sends from a job on the CM box rather than the CD server or is this impossible?