If the machine isn’t a real mail server, you will always have limitations because mail protocols are not designed to work this way
In a “user of the server” (customers) perspective, it’s as mail server because it’s has connected various clients, smartphones, etc… and he “doesn’t care” where the mail come from… He expects that the server makes his “dirty work” and doesn’t have limitations .
There are some situations out there where we have to make some sort of miracles because we don’t have control over all the network structure, the domain registration, the internet connection (ISP that block ports, routers with inaccessible configurations, dynamic IPs), etc…
You know, I’m a sort of the “troll” here that use the “enterprise” version of Nethesis products.
I’m trying to understand where the “wind is blowing” here and what I have to expect in the “enterprise” versions in the future and to make a sort of “lobbying” for what I consider lacking in the product 
(Feel free to delete my last sentences if these may cause problems).