Once Webmin is operational you can download and install the Virtualmin modules and theme in either RPM format (for RPM-based Linux distributions), deb format (for deb-based Linux distributions), or wbm format (for any other UNIX or Linux system), and install them using the Webmin Modules module found in Webmin:Webmin Configuration.
https://software.virtualmin.com/vm/6/gpl/wbm/
virtual-server-7.1.gpl-1.wbm.gz
install any MTA like Postfix, Sendmail or Qmail