[exim] Question on IP origination schema

I'm an Exim newbie and could use some help in defining this network
structure.

Master-Controller.com, for example, is a web hosting company that also
allows users to login and create email campaigns (this is just an example).

Exim, running on Master-Controller.com, interrogates departing messages
and based on the From header (or you suggest what?) sends the message(s)
to the properly registered mx mail server for the domain stated in the
From header (or what you might suggest).

The mx mail server receives the message, sees that it can relay messages
for Master-Controller.com and sends it on it's way to its final destination.

Here's the million dollar question: does the final recipient looking at
the source of the message see that it originated at the IP address for
Master-Controller.com or the mx mailer for the stated domain?

I need it to be for the IP address of the registered mx domain.

thank you.