[exim] Changing email headers to reflect server IP when they…

Hi everyone, I have a private server that I use to host a few websites
and send emails for my company and some of our affiliates. However, we
have an issue with mobile devices that use third party email clients to
send and receive emails. The problem being that when they are in the
field using Verizon LTE or similar that their dynamic ips are
automatically blacklisted in most major DNS lists. I only really have 2
options.

1. Everyone buys a clean static ip (expensive)
2. Rewrite the headers of the email to reflect the server's ip address
(free)

I'm hoping for option two.

Can anyone shed some light on how to accomplish this, if possible?

Thanks,