Services can be monitored and alerts can be issued via Webmin -> Others -> System and Server Status.
I am currently able to offer for free (via a third party service which I use) a way for Virtualmin admins to be alerted via SMS and email when any service on their server is down. Services which can be monitored include Apache, Dovecot and Postfix. The monitoring is done remotely, so no software or agent needs to be installed on the Virtualmin server; all that is needed for monitoring to be enabled is the IP address of the Virtualmin server.
Anyone who wishes to avail of this service is welcome to message me via this forum or Google me, I am very easy to reach.