Their is a lot of complexity to implementing a Zixmail-type solution. The outbound gateway device or function has to intercept the restricted message and redirect it to a web server., Then the web server needs to send the user a replacement message instructing him to connect to the web server to create an account and pick up his mail. On all the known implementations, this is a cloud-based website provided by the application vendor, not an on-premise solution. Up to this point, SmarterTools has not attempted to be a cloud service vendor, and it strikes me as a money-loser for them.
I suggest buying a Barracuda appliance. The appliance is capacity-limited, which is reasonable, rather than per-user-licensed, which gets expensive quickly.