By default, the DNS MX record (which controls where email is delivered)
for new domains will be mail.${DOM}, 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.
You can also use Virtualmin's standard variables like ${DOM} to specify an
MX record with a different name in domain being created.