Are all your DNS servers reporting the same mx record?
Its also good to setup mail.domain prior to adding the domain to the mail server app, including doing the ssl certs as that speeds up refresh ttl waits.
I reported this to the team before, sometimes it doesnt reflect the actual status of correct settings. You may need to refresh the mail app a few times over a few mins to see it working properly. i have noticed this happen with many domains there, just not a single domain, but that could be it too. Keep clicking 'Domain list' every 30 secs and see if that eventually resolves itself, you may have that bug too - that is, aesthetic bug.