This is a comparison of mail servers: mail transfer agents, mail delivery agents, and other computer software that provide e-mail services.
Unix based mail servers are built using a number of components because a Unix style environment is, by default, a toolbox 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. Unix based MTA software largely acts as enhancement or replacement of the respective system's "native" MTA.
Windows servers do not natively implement email. Windows based MTAs therefore have to cover the whole set of email related functionality.
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Source of the article : Wikipedia
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