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CONTENTS

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 * (Top)
 * 1Retrieving messages from a mailbox
 * 2Message composition
 * 3Submitting messages to a server
 * 4Encryption
   Toggle Encryption subsection
   * 4.1Encryption of the message body
 * 5Webmail
 * 6Remote messages
   Toggle Remote messages subsection
   * 6.1Advantages
   * 6.2Disadvantages
 * 7Protocols
   Toggle Protocols subsection
   * 7.1Port numbers
   * 7.2Proprietary client protocols
 * 8See also
 * 9References
 * 10Bibliography

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EMAIL CLIENT

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From Wikipedia, the free encyclopedia

Computer program used to access and manage a user's email
This article is about mail readers for Internet e-mail. For BBS mail readers,
see Offline reader.

Mozilla Thunderbird email client user interface on a Linux operating system

An email client, email reader or, more formally, message user agent (MUA) or
mail user agent is a computer program used to access and manage a user's email.

A web application which provides message management, composition, and reception
functions may act as a web email client, and a piece of computer hardware or
software whose primary or most visible role is to work as an email client may
also use the term.


RETRIEVING MESSAGES FROM A MAILBOX[EDIT]

Like most client programs, an email client is only active when a user runs it.
The common arrangement is for an email user (the client) to make an arrangement
with a remote Mail Transfer Agent (MTA) server for the receipt and storage of
the client's emails. The MTA, using a suitable mail delivery agent (MDA), adds
email messages to a client's storage as they arrive. The remote mail storage is
referred to as the user's mailbox. The default setting on many Unix systems is
for the mail server to store formatted messages in mbox, within the user's home
directory. Of course, users of the system can log-in and run a mail client on
the same computer that hosts their mailboxes; in which case, the server is not
actually remote, other than in a generic sense.

Emails are stored in the user's mailbox on the remote server until the user's
email client requests them to be downloaded to the user's computer, or can
otherwise access the user's mailbox on the possibly remote server. The email
client can be set up to connect to multiple mailboxes at the same time and to
request the download of emails either automatically, such as at pre-set
intervals, or the request can be manually initiated by the user.

A user's mailbox can be accessed in two dedicated ways. The Post Office Protocol
(POP) allows the user to download messages one at a time and only deletes them
from the server after they have been successfully saved on local storage. It is
possible to leave messages on the server to permit another client to access
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.

Alternatively, the Internet Message Access Protocol (IMAP) allows users to keep
messages on the server, flagging them as appropriate. IMAP provides folders and
sub-folders, which can be shared among different users with possibly different
access rights. Typically, the Sent, Drafts, and Trash folders are created by
default. IMAP features an idle extension for real-time updates, providing faster
notification than polling, where long-lasting connections are feasible. See also
the remote messages section below.

The JSON Meta Application Protocol (JMAP) is implemented using JSON APIs over
HTTP and has been developed as an alternative to IMAP/SMTP.

In addition, the mailbox storage can be accessed directly by programs running on
the server or via shared disks. Direct access can be more efficient but is less
portable as it depends on the mailbox format; it is used by some email clients,
including some webmail applications.


MESSAGE COMPOSITION[EDIT]

Email clients usually contain user interfaces to display and edit text. Some
applications permit the use of a program-external editor.

The email clients will perform formatting according to RFC 5322 for headers and
body, and MIME for non-textual content and attachments. Headers include the
destination fields, To, Cc (short for Carbon copy), and Bcc (Blind carbon copy),
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.

Client 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[EDIT]

When a user wishes to create and send an email, the email client will handle the
task. The email client is usually set up automatically to connect to the user's
mail server, which is typically either an MSA or an MTA, two variations of the
SMTP protocol. The email client which uses the SMTP protocol creates an
authentication extension, which the mail server uses to authenticate the sender.
This method eases modularity and nomadic computing. The older method was for the
mail server to recognize 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.

Client 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.


ENCRYPTION[EDIT]

With no encryption, much like for postcards, email activity is plainly visible
by any occasional eavesdropper. Email encryption enables privacy to be
safeguarded by encrypting the mail sessions, the body of the message, or both.
Without it, anyone with network access and the right tools can monitor email and
obtain login passwords. Examples of concern include the government censorship
and surveillance and fellow wireless network users such as at an Internet cafe.

All relevant email protocols have an option to encrypt the whole session, to
prevent a user's name and password from being sniffed. They are strongly
suggested for nomadic users and whenever the Internet access provider is not
trusted.[1] When sending mail, users can only control encryption at the first
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 one.

Encrypted 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.

Encrypting an email retrieval session with, e.g., SSL, can protect both parts
(authentication, and message transfer) of the session.[2][3]

Alternatively, if the user has SSH access to their mail server, they can use SSH
port forwarding to create an encrypted tunnel over which to retrieve their
emails.[4]


ENCRYPTION OF THE MESSAGE BODY[EDIT]

There are two main models for managing cryptographic keys. S/MIME employs a
model based on a trusted certificate authority (CA) that signs users' public
keys. OpenPGP 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. Header fields, including
originator, recipients, and often subject, remain in plain text.


WEBMAIL[EDIT]

Main article: Webmail

In addition to email clients running on a desktop computer, there are those
hosted remotely, either as part of a remote UNIX installation accessible by
telnet (i.e. a shell account), or hosted on the Web. Both of these approaches
have several advantages: they share an ability to send and receive email away
from the user's normal base using a web browser or telnet client, thus
eliminating the need to install a dedicated email client on the user's device.

Some websites are dedicated to providing email services, and many Internet
service providers provide webmail services as part of their Internet service
package. The main limitations of webmail are that user interactions are subject
to the website's operating system and the general inability to download email
messages and compose or work on the messages offline, although there are
software packages that can integrate parts of the webmail functionality into the
OS (e.g. creating messages directly from third party applications via MAPI).

Like IMAP and MAPI, webmail provides for email messages to remain on the mail
server. See next section.


REMOTE MESSAGES[EDIT]

POP3 has an option to leave messages on the server. By contrast, both IMAP and
webmail keep messages on the server as their method of operating, albeit users
can make local copies as they like. Keeping messages on the server has
advantages and disadvantages.[5]


ADVANTAGES[EDIT]

 * Messages can be accessed from various computers or mobile devices at
   different locations, using different clients.
 * Some kind of backup is usually provided by the server.


DISADVANTAGES[EDIT]

 * With limited bandwidth, access to long messages can be lengthy, unless the
   email client caches a local copy.
 * There may be privacy concerns since messages that stay on the server at all
   times have more chances to be casually accessed by IT personnel, unless
   end-to-end encryption is used.


PROTOCOLS[EDIT]

Popular protocols for retrieving mail include POP3 and IMAP4. Sending mail is
usually done using the SMTP protocol.

Another 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.

Most email clients use a User-Agent[6] header field to identify the software
used to send the message. This header field is defined for Netnews, but not-for
e-mail, and, as such, is non-standard[7] in e-mail headers.

RFC 6409, 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."

RFC 5965, An Extensible Format for Email Feedback Reports, provides "an
extensible format and MIME type that may be used by mail operators to report
feedback about received email to other parties."


PORT NUMBERS[EDIT]

Email servers and clients by convention use the TCP port numbers in the
following table. For MSA, IMAP and POP3, the table reports also the labels that
a client can use to query the SRV records and discover both the host name and
the port number of the corresponding service.[8]

Protocol Use Plain text or
encrypt sessions Plain text
sessions only Encrypt sessions
only POP3 incoming mail 110
_pop3._tcp 995
_pop3s._tcp IMAP4 incoming mail 143
_imap._tcp 993
_imaps._tcp SMTP outgoing mail 25 587 MSA outgoing mail 587
_submission._tcp 465[9]
_submissions._tcp HTTP webmail 80 443

While webmail obeys the earlier HTTP disposition of having separate ports for
encrypt and plain text sessions, mail protocols use the STARTTLS technique,
thereby allowing encryption to start on an already established TCP connection.
While RFC 2595 used to discourage the use of the previously established ports
995 and 993, RFC 8314 promotes the use of implicit TLS when available.


PROPRIETARY CLIENT PROTOCOLS[EDIT]

Microsoft mail systems use the proprietary Messaging Application Programming
Interface (MAPI) in client applications, such as Microsoft Outlook, to access
Microsoft Exchange electronic mail servers.


SEE ALSO[EDIT]

Look up email client in Wiktionary, the free dictionary.
 * Comparison of email clients
 * Mail submission agent (MSA)
 * Mailto
 * Message delivery agent (MDA)
 * Message transfer agent (MTA)
 * Simple Mail Transfer Protocol
 * Text-based email client


REFERENCES[EDIT]

 1. ^ C. Hutzler; D. Crocker; P. Resnick; E. Allman; T. Finch (November 2007).
    "Message Submission Authentication/Authorization Technologies". Email
    Submission Operations: Access and Accountability Requirements. IETF. sec. 5.
    doi:10.17487/RFC5068. BCP 134. RFC 5068. Retrieved 24 August 2011. This
    document does not provide recommendations on specific security
    implementations. It simply provides a warning that transmitting user
    credentials in clear text over insecure networks SHOULD be avoided in all
    scenarios as this could allow attackers to listen for this traffic and steal
    account data. In these cases, it is strongly suggested that an appropriate
    security technology MUST be used.
 2. ^ Sill 2003, p. 353: "Like SMTP, POP3 is unencrypted. Unlike SMTP, however,
    it needs authentication: Users have to identify themselves and prove they're
    who they claim to be. Unfortunately, the authentication usually consists of
    presenting a username and a password known only to the user and the POP3
    server. Because the POP3 dialogue is unencrypted, an eavesdropper can obtain
    a user's username and password and reuse them to access the user's mailbox.
    So, plain POP3 exposes the contents of the mail messages the user retrieves,
    and it exposes their username and password, which can then be reused by
    someone else.
    Wrapping the POP3 dialogue with transport-layer security such as SSL solves
    both of these problems. Because SSL-wrapped POP3 sessions are encrypted from
    beginning to end, no messages, usernames, or passwords are exposed in
    cleartext.
    The optional POP3 command, APOP, replaces the standard USER/PASS
    authentication with a challenge-response authentication mechanism. This
    solves the problem of the disclosure of reusable passwords, but does nothing
    to prevent eavesdroppers from reading users' mail messages as they're being
    retrieved."
 3. ^ Updated Transport Layer Security (TLS) Server Identity Check Procedure for
    Email-Related Protocols. doi:10.17487/RFC7817. RFC 7817.
 4. ^ Flickenger, Rob (2003). Linux Server Hacks: 100 Industrial-Strength Tips &
    Tools. O'Reilly Media. p. 146. ISBN 978-0596004613. In addition to providing
    remote shell access and command execution, OpenSSH can forward arbitrary TCP
    ports to the other end of your connection. This can be very handy for
    protecting email, web, or any other traffic you need to keep private (at
    least, all the way to the other end of the tunnel).
    ssh accomplishes local forwarding by binding to a local port, performing
    encryption, sending the encrypted data to the remote end of the ssh
    connection, then decrypting it and sending it to the remote host and port
    you specify. Start an ssh tunnel with the -L switch (short for Local):
    root@laptop:~# ssh -f -N -L110:mailhost:110 -l user mailhost
    Naturally, substitute user with your username, and mailhost with your mail
    server's name or IP address. Note that you will have to be root on the
    laptop for this example since you'll be binding to a privileged port (110,
    the POP port). You should also disable any locally running POP daemon (look
    in /etc/inetd.conf) or it will get in the way.
    Now to encrypt all of your POP traffic, configure your mail client to
    connect to localhost port 110. It will happily talk to mailhost as if it
    were connected directly, except that the entire conversation will be
    encrypted.
 5. ^ "Is IMAP Right for Me?". IT Services. Stanford University. 4 March 2010.
    Retrieved 14 April 2013.
 6. ^ "User-Agent". Netnews Article Format. IETF. November 2009. sec. 3.2.13.
    doi:10.17487/RFC5536. RFC 5536. Some of this information has previously been
    sent in non-standardized header fields such as X-Newsreader, X-Mailer,
    X-Posting-Agent, X-Http-User-Agent, and others
 7. ^ J. Palme (February 1997). "Use of gatewaying headers". Common Internet
    Message Headers. sec. 2. doi:10.17487/RFC2076. RFC 2076. Retrieved May 11,
    2015. Headers defined only in RFC 1036 for use in Usenet News sometimes
    appear in mail messages, either because the messages have been gatewayed
    from Usenet News to e-mail, or because the messages were written in combined
    clients supporting both e-mail and Usenet News in the same client. These
    headers are not standardized for use in Internet e-mail and should be
    handled with caution by e-mail agents.
 8. ^ Cyrus Daboo (March 2011). Use of SRV Records for Locating Email
    Submission/Access Services. IETF. doi:10.17487/RFC6186. RFC 6186. Retrieved
    17 April 2013.
 9. ^ Keith Moore; Chris Newman (January 2018). Cleartext Considered Obsolete:
    Use of Transport Layer Security (TLS) for Email Submission and Access. IETF.
    doi:10.17487/RFC8314. RFC 8314. Retrieved 12 February 2018.


BIBLIOGRAPHY[EDIT]

 * Sill, Dave (2003). The qmail Handbook. Apress. ISBN 9781430211341.
 * Partridge, Craig (April–June 2008). "The Technical Development of Internet
   Email" (PDF). IEEE Annals of the History of Computing. 30 (2): 3–29.
   doi:10.1109/mahc.2008.32. ISSN 1934-1547. S2CID 206442868. Archived from the
   original (PDF) on 2011-05-12.



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