1. I don't see anything about DMARC (DKIM, SPF) setup for your users. Do you provide DMARC?
2. Do you use shared ip's for all your users? If yes, how do you make sure my emails don't land in spam-filters, because of other users behaving badly?
3. Do you have a system in place and already experience to behave differently to different email hosters (e.g. send emails differently to gmail, yahoo or gmx)?
4. Do you provide spam-filters for incoming emails?
5. Do you provide support for email encryption and signatures (might be trivial, because it's part of the client, not sure about this one)?
6. What are your availability and reliability guarantees? What is your average/90%/99% delivery time and how often do you eventually drop an email? Will you inform me, when that happens?
7. How do you store my emails? Is strong encryption in place?
These are questions that I would want to have answered before signing up for such a service and they are the things that distinguish you from simply self-hosting emails for that money. It's also the reasons why I sometimes have to get emails from friends using their own domain from the spam-filter.
Sending emails is way harder than most people think (source: have worked in email infrastructure of a company sending billions of emails per month). Problems come especially, because email response codes are used differently across email hosters and it gets especially tricky when multiple independent users send emails over the same ip.