I haven’t really done this. The closest was multiple instances of PHPList on a vps with different clients all using the same MySql server with their own databases. That doesn’t really fulfill the definition of MTA.
I would have to know more about your intended implementation of a multi tenant architecture in order to give a better answer. I get the impression you are trying to use PHPList to create a service like mailchimp or jangomail.
The first concern that comes to mind is where will the email be sent from? Would it be the servers of the individual tenants, or the centralized app server?
Using the app server would be a bad idea due to the high likelihood of one tenant getting the server placed on spam blocklists. You should use the appserver to initiate the tenant mailservers without appearing to be relayed mail.
A significant amount of review of RDNS, and other authentication methods must be performed. Consider also using SSL certs. You’ll also have to make sure you are on as many FBL’s as possible (and actually monitor them).
The Second concern is the horsepower of the SAAS server. PHPList 3* uses a lot of resources (something I will soon address with the devs). Using cronjobs for each tenant will most likely bring an average server to it’s knees. You should write a shell script that takes care of this upon demand.
A Third concern would be the cluster configuration. All tenant servers should have their own segregated mailserver. Delivery speed is often mission critical and it doesn’t make sense to have any bottlenecks.