You could certainly change the MX and mail A records in your custom domain DNS to route mail somewhere else, but you will find that things get really messy, complicated, and wierd very quickly. (Generally not guaranteed to work.) Generally speaking, if the FTH code detects you are using a custom domain, internally it will expect that domain to be available for the FTH provided email addresses, and handle email accordingly. All the email handling that FTH provides is tightly integrated with the design of the system. It is not really part of the design philosophy for the officer email addresses to be anything other than forwarding email addresses.
Reference the following past threads:
support.toastmastersclubs.org/2011-10-23...ils,-g-suite-and-fth
support.toastmastersclubs.org/2011-10-23...essage-not-delivered