FREE EMAIL SERVER IN LINUX OR WINDOWS

hi
im really thankful with techripublic & members because i solved many problems with techrepublic.

here im asking about the mail server.in my company we are using accounting system through vpn.we have static ip in our head office and all branches can access.we have approximate 20 sales team and thousands of customers.we don’t have website.now we have a plan to implement mail server for communication between us (internal) and customers(external).any free solution for this purpose in Linux or windows and please explain me..