• An email
client, email reader, or more formally mail user agent (MUA), is a computer
program used to manage email.
• The term
email client may refer to any agent acting as a client toward an email server,
regardless of it being a mail user agent, a relaying server, or a human typing
on a terminal.
• A web
application providing message management, composition, and reception
functionality is sometimes considered an email client.
Retrieving messages from a
• Like most
client programs, an MUA is only active when a user runs it. Messages arrive on
the Mail Transfer Agent (MTA) server.
the MUA has access to the server's disk, messages are stored on a remote server
and the MUA has to request them on behalf of the user.
• In the
first case, shared disk, a user logs on a server and runs an MUA on that
machine. The MUA reads messages from a conventionally formatted storage,
typically mbox, within the user's HOME directory.
• The MTA
uses a suitable mail delivery agent (MDA) to add messages to that storage,
possibly in concurrence with the MUA. This is the default setting on many UNIX
systems. Web mail applications running on the relevant server can also benefit
from direct disk access to the mail storage.
personal computing, and whenever messages are stored on a remote system, a mail
user agent connects to a remote mailbox to retrieve messages.
remote mailboxes comes in two flavors. On the one hand, the Post Office
Protocol (POP) allows the client to download messages one at a time and only
delete them from the server after they have been successfully saved on local
storage. It is possible to leave messages on the server in order to let another
client download them. However, there is no provision for flagging a specific
message as seen, answered, or forwarded, thus POP is not convenient for users
who access the same mail from different machines or clients.
• On the
other hand, the Internet Message Access Protocol (IMAP) allows users to keep
messages on the server, flagging them as appropriate. IMAP provides
sub-folders. Typically, the Sent, Drafts, and Trash folders are created by
• Both POP
and IMAP clients can be configured to access more mailboxes at the same time,
as well as to check each mailbox every given number of minutes. IMAP features
an idle extension for real time updates, providing faster notification than
polling where long lasting connections are feasible.
settings require the server's name or IP address, and the user name and
password for each remote incoming mailbox.
• Mail user
agents usually have built-in the ability to display and edit text. Editing HTML
text is a popular feature. Invoking an external editor may be an alternative.
responsibilities include proper formatting according to RFC 5322 for headers
and body, and MIME for non-textual content and attachments.
include the destination fields, To, Cc, and Bcc, and the originator fields from
which is the message's author(s), Sender in case there are more authors and
Reply-To in case responses should be addressed to a different mailbox.
• To better
assist the user with destination fields, many clients maintain one or more
address books and/or are able to connect to an LDAP directory server. For
originator fields, clients may support different identities.
settings require the user's real name and email address for each user's
identity, and possibly a list of LDAP servers.
Submitting messages to a server
• An MUA is
able to introduce new messages in the transport system. Typically, it does so
by connecting to either an MSA or an MTA, two variations of the SMTP protocol.
client needs to put a message quickly without worrying about where the message
eventually will be delivered: that's why a transport system exists. Thus it
always connects to the same preferred server, however, how does that server
know that it should accept and relay submissions from that client.
• There are
two ways. The older method recognizes the client's IP address, e.g. because the
client is on the same machine and uses internal address 127.0.0.1, or because
the client's IP address is controlled by the same internet service provider
that provides both internet access and mail services.
• The newer
method, since the SMTP protocol has an authentication extension, is to
authenticate. The latter method eases modularity and nomadic computing.
settings require the name or IP address of the preferred outgoing mail server,
the port number (25 for MTA, 587 for MSA), and the user name and password for
the authentication, if any.
• There is
a non-standard port 465 for SSL encrypted SMTP sessions, that many clients and
servers support for backward compatibility. Transport Layer Security encryption
can be configured for the standard ports, if both the client and the server
it, anyone (examples: the government (warrantless wiretapping, great firewall
of China), fellow wireless network users such as at an Internet cafe or other
public network, whether the network is open or not) with network access and the
right tools can monitor email and obtain login passwords.
Encryption of mail sessions
relevant email protocols have an option to encrypt the whole session.
Remarkably, those options prevent a user's name and password from being
sniffed, therefore they are recommended for nomadic users and whenever the
internet access provider is not trusted.
sending mail, users can only control encryption at the hop from a client to its
configured outgoing mail server. At any further hop, messages may be
transmitted with or without encryption, depending solely on the general
configuration of the transmitting server and the capabilities of the receiving
mail sessions deliver messages in their original format, i.e. plain text or
encrypted body, on a user's local mailbox and on the destination server's. The
latter server is operated by an email hosting service provider, possibly a
different entity than the internet access provider currently at hand.
Encryption of the message body
• There are
two models for managing cryptographic keys. S/MIME employs a model based on a
trusted certificate authority (CA) that signs users' public keys.
employs a somewhat more flexible web of trust mechanism that allows users to
sign one another's public keys. OpenPGP is also more flexible in the format of
the messages, in that it still supports plain message encryption and signing as
they used to work before MIME standardization.
• In both
cases, only the message body is encrypted. Headers, including originator,
recipients, and subject, remain in plain text.
popular protocols for retrieving mail include POP3 and IMAP4, sending mail is
usually done using the SMTP protocol.
important standard supported by most email clients is MIME, which is used to
send binary file email attachments. Attachments are files that are not part of
the email proper, but are sent with the email.
email clients use an X-Mailer header to identify the software used to send the
message. According to RFC 2076, this is a common but non-standard header.
• RFC 4409,
Message Submission for Mail, details the role of the Mail submission agent.
• RFC 5068,
Email Submission Operations: Access and Accountability Requirements, provides a
survey of the concepts of MTA, MSA, MDA, and MUA. It mentions that "Access
Providers MUST NOT block users from accessing the external Internet using the
SUBMISSION port 587" and that "MUAs SHOULD use the SUBMISSION port
for message submission."
application that runs on a personal computer or workstation and enables you to
send, receive and organize e-mail. It's called a client because e-mail systems
are based on a client-server architecture. Mail is sent from many clients to a
central server, which re-routes the mail to its intended destination.