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{MAJOROS} offers many advanced applications to serve and access email. This chapter describes modern email protocols in use today, and some of the programs designed to send and receive email.
Email Protocols
indexterm:[email,protocols] Today, email is delivered using a client/server architecture. An email message is created using a mail client program. This program then sends the message to a server. The server then forwards the message to the recipient's email server, where the message is then supplied to the recipient's email client.
To enable this process, a variety of standard network protocols allow different machines, often running different operating systems and using different email programs, to send and receive email.
The following protocols discussed are the most commonly used in the transfer of email.
Mail Transport Protocols
Mail delivery from a client application to the server, and from an originating server to the destination server, is handled by the _Simple Mail Transfer Protocol_ (_SMTP_).
====== SMTP indexterm:[email,protocols,SMTP] The primary purpose of SMTP is to transfer email between mail servers. However, it is critical for email clients as well. To send email, the client sends the message to an outgoing mail server, which in turn contacts the destination mail server for delivery. For this reason, it is necessary to specify an SMTP server when configuring an email client.
Under {MAJOROS}, a user can configure an SMTP server on the local machine to handle mail delivery. However, it is also possible to configure remote SMTP servers for outgoing mail.
One important point to make about the SMTP protocol is that it does not require authentication. This allows anyone on the Internet to send email to anyone else or even to large groups of people. It is this characteristic of SMTP that makes junk email or _spam_ possible. Imposing relay restrictions limits random users on the Internet from sending email through your SMTP server, to other servers on the internet. Servers that do not impose such restrictions are called _open relay_ servers.
{MAJOROS} provides the Postfix and Sendmail SMTP programs.
Mail Access Protocols
There are two primary protocols used by email client applications to retrieve email from mail servers: the _Post Office Protocol_ (_POP_) and the _Internet Message Access Protocol_ (_IMAP_).
====== POP indexterm:[email,protocols,POP] The default POP server under {MAJOROS} is [application]*Dovecot* and is provided by the [package]*dovecot* package.
Installing the dovecot package
In order to use [application]*Dovecot*, first ensure the [package]*dovecot* package is installed on your system by running, as `root`:
~]#{nbsp}dnf install dovecot
For more information on installing packages with DNF, see xref:../package-management/DNF.adoc#sec-Installing[Installing Packages].
When using a `POP` server, email messages are downloaded by email client applications. By default, most `POP` email clients are automatically configured to delete the message on the email server after it has been successfully transferred, however this setting usually can be changed.
`POP` is fully compatible with important Internet messaging standards, such as _Multipurpose Internet Mail Extensions_ (_MIME_), which allow for email attachments.
`POP` works best for users who have one system on which to read email. It also works well for users who do not have a persistent connection to the Internet or the network containing the mail server. Unfortunately for those with slow network connections, `POP` requires client programs upon authentication to download the entire content of each message. This can take a long time if any messages have large attachments.
The most current version of the standard `POP` protocol is `POP3`.
There are, however, a variety of lesser-used `POP` protocol variants:
*APOP* — `POP3` with `MD5` authentication. An encoded hash of the user's password is sent from the email client to the server rather than sending an unencrypted password.
*KPOP* — `POP3` with Kerberos authentication.
*RPOP* — `POP3` with `RPOP` authentication. This uses a per-user ID, similar to a password, to authenticate POP requests. However, this ID is not encrypted, so `RPOP` is no more secure than standard `POP`.
For added security, it is possible to use _Secure Socket Layer_ (_SSL_) encryption for client authentication and data transfer sessions. This can be enabled by using the [command]#pop3s# service, or by using the [command]#stunnel# application. For more information on securing email communication, see xref:Mail_Servers.adoc#s2-email-security[Securing Communication].
====== IMAP indexterm:[email,protocols,IMAP] The default `IMAP` server under {MAJOROS} is [application]*Dovecot* and is provided by the [package]*dovecot* package. See xref:Mail_Servers.adoc#s3-email-protocols-pop[POP] for information on how to install [application]*Dovecot*.
When using an `IMAP` mail server, email messages remain on the server where users can read or delete them. `IMAP` also allows client applications to create, rename, or delete mail directories on the server to organize and store email.
`IMAP` is particularly useful for users who access their email using multiple machines. The protocol is also convenient for users connecting to the mail server via a slow connection, because only the email header information is downloaded for messages until opened, saving bandwidth. The user also has the ability to delete messages without viewing or downloading them.
For convenience, `IMAP` client applications are capable of caching copies of messages locally, so the user can browse previously read messages when not directly connected to the `IMAP` server.

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  2. f26/pages/servers/Mail_Servers
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In order to use [application]*Dovecot*, first ensure the [package]*dovecot* package is installed on your system by running, as `root`:
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