Overview of the mail server architecture
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The mail server infrastructure consists of several components that work together to send, relay, receive, store, and deliver email.
The mail server workload uses the following Internet standard protocols for sending and retrieving email:
  • Simple Mail Transfer Protocol (SMTP): the Internet standard protocol for sending email.
  • Post Office Protocol (POP): an Internet standard protocol for retrieving email.
  • Internet Message Access Protocol version 4 (IMAPv4): an Internet standard protocol for retrieving email.
The following table lists each mail server component, describes each component, and provides some examples of each component.
Table 1. Mail server components, descriptions, and examples
Component Description Examples
Mail User Agent (MUA) An application with which users can create, view, send, and receive email. The MUA is located on a client system, such as a workstation or PC.
  • Microsoft Outlook Express
  • Mozilla Thunderbird
  • Mutt E-Mail Client
Mail Transfer Agent (MTA) An application that sends, receives, and stores email. This program determines where and how to store email.
  • Postfix
  • Sendmail
  • Lotus Domino Server
  • Microsoft Exchange
Mail Delivery Agent (MDA) An application that saves received email to the MSA. This program might also perform additional tasks such as filtering email or delivering email to subfolders.
The Postfix, Dovecot, and Cyrus applications each implement some or all of the functions of the MDA.
Mail Storage Area (MSA) A local system or server where the MTA stores email. This is also the location from which the MSS retrieves email at the request of the MUA.
  • Mbox
  • Maildir
  • /var/mail/spool/username/
Mail Storage Server (MSS) An application that retrieves email from the MSA and returns it to the MUA.
  • Dovecot
  • Cyrus
The following figure shows the mail server components and the flow of email through those components.
Mail server architecture
Email flows through the mail server components as follows:
  1. From their MUA, the sender creates an email and clicks Send.
  2. The MUA uses SMTP to send the email to an MTA.
  3. The MTA relays and routes the email to an MTA in the domain of the recipient.
  4. The MTA in the domain of the recipient sends the email to an MDA of the system of the recipient.
  5. The MDA stores the email in an MSA.
  6. The MUA of the recipient queries an MSS.
  7. The MSS uses IMAPv4 or POP to retrieve the email for the recipient from the MSA.
  8. The MSS returns the email to the MUA.
  9. From their MUA, the recipient reads the email created by the sender.
In conjunction with mail server applications and email clients, you can use additional applications to pre-process and post-process email. For example, you can use filtering applications, anti-virus software, or anti-spam applications. A description of deploying such applications is outside the scope of this document. You may choose to select solutions to work in conjunction with the mail server workload for such additional functions.

Postfix overview

Postfix is an application that provides Simple Mail Transfer Protocol (SMTP) and Mail Transfer Agent (MTA) functions. Postfix is included in the mail server workload for PowerLinux servers.

Dovecot overview

Dovecot is an application that provides Post Office Protocol (POP), Internet Message Access Protocol version 4 (IMAPv4), and Mail Storage Server functions. Dovecot is included in the mail server workload for PowerLinux servers.

Cyrus overview

Cyrus is an application that provides Post Office Protocol (POP), Internet Message Access Protocol version 4 (IMAPv4), and Mail Storage Server functions. Cyrus is included in the mail server workload for PowerLinux servers.