Howto Provide Backup MX Service for Clients

Hi,

We currently have a Virtualmin Pro (primary) server and a Virtualmin GPL server (rented VPS) and all is working well. The secondary Virtualmin GPL server provides backup services for DNS, MX, LDAP, RADIUS, etc.

What I want to do is easily provide backup MX service, and/or spam filtering for my customers that have their own mail servers and sell this as a monthly or annual service.

I understand this requires a secondary MX record in the customer's DNS...no problem.

I think all it would take is adding a domain to the Postfix relay_domains parameter:

postconf -e 'relay_domains = $mydestination, newdomain.com'

To provide a spam filtering service, I could probably just firewall off the primary MX, except from my mail server (the backup MX + spam filter).

Is there an easy way to do this within Virtualmin or Webmin?

Thank in advance,

G