A beginner’s guide to running your own mail server is available here: “NSA-proof your e-mail in 2 hours”.
Kolab integrates Roundcube into its webclient and offers desktop clients as well. Recent versions also feature a file cloud turning it into a complete solution for personal information management.
What is an MTA?
“Within Internet message handling services (MHS), a message transfer agent or mail transfer agent (MTA) or mail relay is software that transfers electronic mail messages from one computer to another using a client–server application architecture. An MTA implements both the client (sending) and server (receiving) portions of the Simple Mail Transfer Protocol.
The terms mail server, mail exchanger, and MX host may also refer to a computer performing the MTA function. The Domain Name System (DNS) associates a mail server to a domain with mail exchanger (MX) resource records containing the domain name of a host providing MTA services.”