Turnkey Linux
Open Source
Turnkey Linux Virtual Appliance Library is a free open source project which has developed a range of Debian based pre-packaged server software.
Microsoft Exchange Server
Commercial
Microsoft Exchange Server is a mail server and calendaring server.
Compare Zentyal with alternatives
Compare Turnkey Linux and Zentyal and decide which is most suitable for you.
Compare Microsoft Exchange Server and Zentyal and decide which is most suitable for you.