A mail server consists of the following components:
A storage area where email data is stored for local users.
A set of user defined rules that determine how the mail server should react to the destination of a specific message.
A database of user accounts that the mail server recognizes and deals with locally.
Communications modules, which are the components that actually handle the transfer of messages to and from other mail servers and email clients.
A mail server may provide many functionality such as messaging, calendar, tasks, contacts or address books, meetings, and appointments.
You can use various email protocols to access email messages stored in mail servers. You can also use the MAPI protocol to access data stored in a Microsoft Exchange Server or an Outlook Personal Storage Table (PST) file.