I agree with much of what is said. I have no plan to bring email services for a domain back onto the web-hosting server. This is one of the primary reasons we decided to take a chance on Enhance last year. I wanted something similar to what I had with Interworx years ago, or with our customized setup, where mail services/delivery were running on separate hardware/vm. Enhance is the first platform that makes this process simple. We used Plesk Expand many years ago, until they dead-ended and dumped it. We then utilized Interworx until some major changes happened with programmers, and the project nearly stalled for several years. We then moved back to Plesk and used separate SmarterMail servers... but management for that, as well as big-time costs for SmarterMail in excess of 1K email boxes made us move mail back to the Plesk servers... which in turn created new headaches.
The way Enhance allows us to separate services and easily manage it from a centralized control server is exactly what we needed. Now, the mail was barely rudimentary at that time. It is getting better, but eventually we will need queue management (I know, on the roadmap); mail logs in the interface (sorry, I am tired of logging into each mail server over and over), real rock solid mail quota management for mail on remove servers... SIMPLE spam management for end users/resellers.
So, it is going to be important that at some point the Enhance team allow us to set hard limits on mail storage on the dedicated mail servers. Right now, we do a rather manual process each month, but since we are a more boutique offering, we do not have the volume of customers many of your companies may require. I cannot imagine trying to do this with thousands of customers x however many mailboxes each may have.
It is workable at our customer load, but not indefinitely.
Also, side note, we never allow our customers to use mail delivery that is not authenticated for their various web forms/delivery. We stopped that stupidity 20 years ago. Yes, some customers who have used go daddy level hosting complain when they first come on-board, but I am not letting a hacked/improperly setup web form to blindly send email through phpmail without any real checks an balances. I realize not every host wants it that way, but I want a sure-fire log showing authentication, and mail delivery data so we can clamp down or fix problem much more easily.
This also makes it easy to clamp # emails per hour on domains relative to that customer's needs.