The popular Ubuntu Linux, famous for its polished, easy-to-use desktop version, also has a specialized Ubuntu server edition.This is more than a re-packaged Ubuntu Desktop; it comes with a kernel tuned for server duties, a full complement of server software and no graphical interface. This is a serious, lean mean server operating system.
Ubuntu Server does little handholding, so it's not for novice system administrators. Rather, it is for the experienced sysadmin who wants maximum control. It includes a number of enterprise-ready features such as AppArmor for security, private cloud, public cloud, high availability, a Landscape client for systems management, Mac OS X and Microsoft network integration, and virtualization.
The installer serves up a number of useful package groups such as Virtual machine host, Samba file server, LAMP stack, Printer server, and Mail server. The Mail server group installs Postfix (SMTP), and the excellent Dovecot for POP3 and IMAP4, which is a stout foundation for an on-site email server. Then add whatever additional services you want, such as a Webmail server, calendaring, shared contacts, discussion boards, file shares, and so on. Or you can install a prefab groupware suite on it.
Ubuntu Server is free of cost, and various commercial support and systems management options are available.