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Ubuntu Unleashed 2013 Edition: Covering 12.10 and 13.04, Eighth Edition by Matthew Helmke

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The Mail Delivery Agent

SMTP is a server-to-server protocol that was designed to deliver mail to systems that are always connected to the Internet. Dial-up systems connect only at the user’s command; they connect for specific operations, and are frequently disconnected. To accommodate this difference, many mail systems also include a mail delivery agent (MDA). The MDA transfers mail to systems without permanent Internet connections. The MDA is similar to an MTA (see the following note), but does not handle deliveries between systems and does not provide an interface to the user.

Note

Procmail or Spamassassin are examples of MDAs; both provide filtering services to the MDA while they store messages locally and then make them available to the ...

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