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Chapter 10. Introduction to Email protocols

An email message is delivered using a client/server architecture. A mail client program creates an email message that is sent to a server. The message is forwarded by the server to the recipient’s email server, which is then forwarded to the recipient’s email client.

The most commonly used protocols in the transfer of email are classified as :

  • Mail Transport Protocol

    • SMTP
  • Mail Access Protocol

    • POP
    • IMAP

This chapter describes the SMTP, POP, and IMAP protocols.

10.1. SMTP

Simple Mail Transfer Protocol (SMTP) administers the mail delivery from a client application to the server, and from an originating server to the destination server. In RHEL, a user can configure an SMTP server on the local machine to administer mail delivery. You can configure remote SMTP servers for outgoing mail.

In RHEL 8, SMTP is by default TLS secured. You can enforce relay restrictions that will limit random users on the Internet from sending email through your SMTP server to other servers.

The SMTP programs, Sendmail and Postfix are available through the AppStream and BaseOS repositories respectively.

10.2. POP

Post Office Protocol (POP) is used by email client applications to retrieve email from mail servers. In a POP server, emails are downloaded by email client applications. POP is compatible with internet messaging standards like Multipurpose Internet Mail Extensions (MIME). Dovecot is the default POP server and is provided by the dovecot package.

The Secure Socket Layer (SSL) encryption enhances the security by client authentication and data transfer sessions in POP.

To enable SSL encryption, use:

  • The POP3 service
  • The stunnel application
  • The starttls command

10.3. IMAP

To organize and store emails, the Internet Message Access Protocol (IMAP) client applications create, rename, or delete mail directories on the server. IMAP proves beneficial for users accessing their emails using multiple machines. IMAP client applications cache copies of messages locally, this allows users to browse previously read messages while not connected to the IMAP server. IMAP is compatible with internet messaging standards like Multipurpose Internet Mail Extensions (MIME).

To enhance the security of the IMAP server you can use Secure Socket Layer (SSL) encryption for client authentication and data transfer sessions. Enable the imaps service or use the stunnel program for the added security.

In RHEL, Dovecot is the default IMAP server and is provided by the dovecot package.