I’m not sure about that, but usually when I create a new virtual server I leave everything as default even though I don’t use the email feature.
The important thing is that I have enabled the Mail for domain feature in Features and Plugins, but it still does not appear when modifying the virtual server.
The only thing I can think of is that you’re trying to enable it for the domain name that matches the hostname.
However, the upcoming Virtualmin 7.30 release will have this option displayed to avoid confusion. Yet, still, it will fail when enabling mail feature for a domain name that matches the hostname.
This can only be overcome by using the Virtualmin CLI, though it’s not recommended.
It seems that you are guessing completely correctly, the server name with the virtual server is probably the same name and the problem happened without my knowledge.
Since I hadn’t used the Mail for Domain feature before, I didn’t notice this at all.