Procmail or Spamassassin are examples of MTAs; both provide filtering services to the MTA while they store messages locally and then make them available to the MUA or email client for reading by the user.
The MDA uses the POP3 or IMAP protocols for this process. In a manner similar to a post office box at the post office, POP3 and IMAP implement a "store and forward" process that alleviates the need to maintain a local mail server if all you want to do is read your mail. For example, dial-up Internet users can intermittently connect to their ISP's mail server to retrieve mail using Fetchmailthe MDA recommended by Ubuntu (see the section "Using Fetchmail to Retrieve Mail" later in this chapter).
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