Good day
I have senior system admins in my team who are able to manage your ISPConfig/Postfix server.
In area of providing Managed Services we have experience with such technologies as Virtuozzo, different types of virtualization (OpenVZ, KVM, XEN), different distributives of OS Linux / OS Unix / OS MS Windows Server systems, VPN/DNS/IPTables/Firewalls/web servers/mail servers configuration and tuning etc.
If you are interested let's discuss all the points in a chat.