Their claims basically are that hard drives are cheap and cost drops and that domains and users are simply a configuration change (which is automated).
They're charging based on something that actually has an impact. Sending emails incurs in a load as well as receiving one back (you probably are getting a response) and work that the emails may incur. The more emails you send, the odds of needing a clean IPs in case something happens (blacklist is the first thing that comes to mind) on the amount of emails they send, dealing with blacklists is something that statistically will happen the more emails are sent as well. So they charge you by the impact each email sent has and for the cost of hard drives.
I guess they're charging more on a probability of the time they need to work on it the more emails are sent. There is a cost associated with developing the code, sure, but the more people, the less it is per person and once it's developed, it's only new features and maintaining.
It seems they've set it up to scale and it seem they have a lot automated. I'm assuming this based on this:
According to our trials, it would take us up to 30 minutes to setup a completely new infrastructure and get all clients back live.
According to the affiliate link, you get up to 50% of what a user you refer pays (if you give a discount, it comes out of your share of the price). So it seems like they've made the math and it looks like they can at least break even, ideally they still see some revenue, with only earning half of what they charge.
It seems very well structured and organized. I'm testing it out. Paid for a year. If I transfer everything (a couple of domains, but only a handful of accounts. Maybe I will end up setting them via aliases...) I may upgrade just to give back. I had been looking into OVH, Rackspace, FastMail, Google Apps, etc and hadn't switched because they charge for everything...