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.
<snip>

If I recall, in Exchange it is possible to set up an address that all mail sent to an account is forwarded to.  After a short search, wWhile not what I remember (I was working with a much older version of Exchange), this article (https://www.bursky.net/index.php/2012/03/copy-emails-on-exchange-to-another-mailbox/) appears to cover a version of that scenario (I presume that it will allow sending to an address outside the organisation).

If using Exim in front of an Exchange server, you could create a custom router to handle this (using "unseen deliver [email address to copy to]") - see https://serverfault.com/questions/299349/exim-how-to-deliver-locally-and-send-a-copy-to-another-server.  I presume there is a way to do so with Postfix, but I'm less familiar with that.

I'd say doing it on Exchange would be the easier of the two options.  Try it with a test mailbox first, the last thing you want is a Director on your back because something went wrong and they didn't get an important email while you were trying to get it working...

Thanks

Gavin