This is extremely minor but when a new domain is created its “owner” account has email enabled by default. There is a checkbox to disable it (Virtualmin → a domain → Edit Users → the owner account). Is there a setting to have it not be enabled in the first place or a way to check the “No” box via the command line after creation?
One reason for email for that account are reported issues for that domain and the system uses that email address. So you will need to point to a legit account.