[bitfolk] Hivemind - MITM email for delivery to multiple servers?

Folks, I've been tasked with finding a way for our directors emails to be delivered to two servers (somewhat simultaneously). They use their "work email addresses" as personal accounts, as well as business accounts (don't get me started), and they don't want to be reliant on connectivity to the corporate Exchange servers, so I've set up a Docker mail server on their home network, but getting their mail delivered to both that MTA and the Exchange servers is above my knowledge grade. I've patiently tried to explain that "that's not how SMTP works", but I'm being met with recalcitrance and "you need to be a solutions provider", so I thought I'd put this in front of the hivemind and see if any of you know of an easy way to do this? Bit of background to help - the only way that I can think of possibly doing this is to MITM it (stick another MTA server outside our network, have it send the emails for the directors to two destination servers and just pass all the other mail to our Exchange infrastructure, if that's even possible without darkweb solutions), and I won't be allowed to use a cloud email service to do this as they are vehemently opposed to "The Cloud" (again, don't get me started). Kind regards Murray Crane