How to change IP in email header?

Question

Email

There is a server with the MEDOC program (reporting to state bodies). There are several enterprises on MEDka. Reporting is sent through the MEDka mail client. For each enterprise, it was created on a mailer, such as gmail, its own mailbox through which reports are sent and notifications are received from the counter. organs. It is necessary that in the header of letters with reports for each enterprise there should be a unique client-ip. I would not like the option with VPNs.