Thank you for your short summarize.
I also did install Mailcow one year ago just for testing in a separate standalone Ubuntu-VM and it worked fine except a few small issues. Later I ran into trouble with the reverse Proxy on my UCS after an update of Mailcow.
Because I only have one public IP-address I needed to forward external requests to Mailcow via UCS.
Incoming mails became filtered by ASSP and forwarded to mailcow and outgoing mails were sent directly from Mailcow via SMTP-gateway at my provider. The client access from outside to mailcow made a few problems, but I didn’t solve it finally, because it only was for testing.
It would be great, if Univention would implement Mailcow as an Univention app, but it just seems to be a nice dream…