[exim] exim as a load balancer

I wonder if this might be possible with Exim?

When we send out a newsletter to our subscribers, it tends to slow down our
regular email from clients on our domain. I have other stand alone servers I
could use as MTAs to share the burden when there's a spike in demand. Due to
the constraints of our newsletter software and other infrastructure, what
I'd like to do is this:

Is it possible to have one Exim be the single MTA that the newsletter
software talks to and have this server then act as a 'load balancer' or at
least a 'round robin', by relaying to 3 other servers that do the actual
delivery?

I'd appreciate if anyone knows if this is possible.

thanks....

PS. I really do need to do it this way if possible, due to some other
constraints.