A mail server, also known as e-mail server is a machine within your network that is responsible for handling all the incoming and outgoing emails. So in short, it acts like a virtual post office. This server stores all the emails for local users as well as have set of defined rules on how to reach the destination of messages, transfer messages internally to specified users and email clients.
When the user requests his or her e-mail, contact is established with the mail server, which then delivers all stored to the client's computer.
Following are some of the popular Open source Email Servers that you can go for
Zimbra is an enterprise-class email, calendar and collaboration solution built for the cloud, both public and private. With a redesigned browser-based interface, Zimbra offers the most innovative messaging experience available today, connecting end users to the information and activity in their personal clouds. Zimbra is considered as an alternative to Microsoft Exchange Server.
Open-Xchange comes in several versions but only the Community version is free. There are other versions like Hosting and Server Edition, and Advanced Server Version. The Advanced Server edition allows you to work with Microsoft Outlook or MS Exchange. The Hosting version is designed for hosting services and ISPs to provide email and group services. Only the Community version is free.
Horde is a free, enterprise ready, browser-based communication suite. It offers a wide range of features and modules to add-in. Users can send, read and organize email messages and manage and share calendars, contacts, tasks and notes with users both inside and beyond their own network. It features different interfaces for different devices, and synchronization out of the box.
Because of many options, it may be difficult for inexperienced users to set up, but most of the administration is performed via a web interface.
For system administrators with experience, Ubuntu Server includes a full range of features such as private and public clouds, security from AppArmor, virtualization, integration between MacOS X and Microsoft Network. The Mail Server includes SMTP, POP3, and IMAP 4 capabilities, as well as Webmail, contact sharing, file sharing, and discussion boards. Ubuntu Server is free and support options are available.
Scalix is best known for its excellent Outlook user experience. It is the current incarnation of HP OpenMail, and has all the bells and whistles Exchange users have come to expect, including group calendaring, PIM, email, and busy/free services. Offer a high-value Outlook experience with push email, PIM, group calendaring, free/busy services, public folders, delegation, and more.
There is both a free community edition and a commercial (supported) version.