@JOduMonT an email server (without email relays) requires a public IP. There is no way around this. Even if you add a second IP, the second IP will be exposed and one can always access your Cloudron dashboard via that IP address. Just have to put an entry in /etc/hosts on my laptop and point that floating IP to my.domain.com and that's it.
I guess you are looking for a way to "sandbox" mail server to a specific IP somehow. We don't have a way to do this in Cloudron. I can move this to Feature Requests.
But also, it might be easier to just create another VM and run mail on the other VM? Security wise , this is the easiest and cleanest instead of writing a lot of code to make sure mail server and dashboard despite being on the same server don't step on each other (networking wise).