16.2 About Email Protocols

16.2.1 About SMTP
16.2.2 About POP and IMAP

Several different network protocols are required to deliver email messages. These protocols work together to allow different systems, often running different operating systems and different email programs, to send, transfer, and receive email.

16.2.1 About SMTP

The Simple Mail Transfer Protocol (SMTP) is a transport protocol that provides mail delivery services between email client applications and servers, and between the originating server and the destination server. You must specify the SMTP server when you configure outgoing email for an email client application.

SMTP does not require authentication. Anyone can use SMTP to send email, including junk email and unsolicited bulk email. If you administer an SMTP server, you can configure relay restrictions that limit users from sending email through it. Open relay servers do not have any such restrictions. Both Postfix and Sendmail are SMTP server programs that use SMTP. Unless you own a domain in which you want to receive email, you do not need to set up an SMTP server.

16.2.2 About POP and IMAP

The Post Office Protocol (POP) is an email access protocol that email client applications use to retrieve email messages from the mailbox on a remote server, typically maintained by an Internet Service Provider (ISP). POP email clients usually delete the message on the server when it has been successfully retrieved or within a short time period thereafter.

The Internet Message Access Protocol (IMAP) is an email access protocol that email client applications use to retrieve email messages from a remote server, typically maintained by their organization. The entire message is downloaded only when you open it, and you can delete messages from the server without first downloading them. Email is retained on the server when using IMAP.

Both POP and IMAP allow you to manage mail folders and create multiple mail directories to organize and store email.

The dovecot package provides the dovecot service that implements both an IMAP server and a POP server.

By default, the dovecot service runs IMAP and POP together with their secure versions that use Secure Socket Layer (SSL) encryption for client authentication and data transfer sessions. The IMAP and POP servers provided by dovecot are configured to work as installed. It is usually unnecessary to modify the configuration file, /etc/dovecot.conf.

For more information, see the dovecot(1) manual page and /usr/share/doc/dovecot-version.