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.