Perhaps this topic is related to the split domain I'm thinking for my next test, please let me elaborate.
I have a Windows network where two Exchange servers in the same Active Directory domain manage several email domains. Enter Nethserver. While testing and, next, migrating users between the Windows servers and Nethserver, the three mail servers need to handle the same multiple domains, perhaps all of them.
In my tests in Zentyal, I've succeeded by disabling the single smart host and setting up postfix transport maps and "external account aliases" as explained in https://wiki.zentyal.org/wiki/Mail_Split_Domain (actually, I've just edited in /etc/postfix/transports, because AFAIK Zentyal does not overwrite it).
The link @davidep has just provided seems very similar to me, although I don't immediately grasp the template part. Am I right, or is this something different? What about the "external account aliases" in that how-to, reportedly needed to let postfix know that such recipients exist? (indeed it didn't work without them).
I assume nothing changes for the Exchange to Nethserver direction.