Hostname for MX record
By default, the DNS MX record (which controls where email is delivered) for new domains will be mail.$DOMAIN, which will be assigned the same IP address as the virtual server's website. However, you can use this field to specify a different mail server hostname - this is typically used to have a consistent MX record across all domains. Be sure to enter a hostname that resolves to your Virtualmin system though, or else email will not be delivered.