Unix-based mail servers are built using a number of components because a Unix-style environment is, by default, a toolbox[1] operating system.
A stock Unix-like server already has internal mail; more traditional ones also come with a full MTA already part of the standard installation.
To allow the server to send external emails, an MTA such as Sendmail, Postfix, or Exim is required.
Mail is read either through direct access (shell login) or mailbox protocols like POP and IMAP.
Microsoft Windows servers do not natively implement e-mail, thus Windows-based MTAs have to supply all the necessary aspects of e-mail-related functionality.