The comparison of mail servers covers 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 [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 . Unix based MTA software largely acts as enhancement or replacement of the respective system's "native" MTA. Microsoft Windows servers do not natively implement email. Windows-based MTAs therefore have to cover the whole set of email-related functionality. Contents 1 Feature comparison 2 Authentication 3 Antispam features 4