It would certainly be nice to have more information on what exactly happened.
But what can you do? You can't police the messages for every potential form of abuse. (I've only ran an automated spam filter to make sure that the service is never used for blatant spam. I've also blanket-blocked some domains whenever I noticed a pattern in any abuse reports, and finally recipients were able to easily do a permanent block themselves). In any case, running a messaging service even more difficult if you're a small guy and not Facebook or Google.
I've added more information about the details of the suspension in another post on here: https://news.ycombinator.com/item?id=24998922
Seriously, cut the ability to reply to emails and that should be fine.
There's no use case to send replies for an anti-spam. Never seen a registration process that required to reply to complete the registration.
Tip: The google postmaster tool can show you the reputation of your domain and how much of your outbound emails are going to spam. That shall give you an idea how well it's abused. https://www.gmail.com/postmaster/
The service has been sending out emails since about 2013. It only lets out a limited number of emails, and there's an anti abuse policy in place. The IP address always has a good reputation with Google and Microsoft, I am well aware of all the feedback loops.