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Linux Mail User Agents (MUA's): e-Mail client software

Internet mail is broken down into three basic parts of which this tutorial will cover Linux e-mail client applications.


  1. MTA or Mail Transfer Agents - sendmail, qmail, etc - Links/Info
  2. Mail Retrieval Software - pop3, imap, etc - Links/Info
  3. Mail User Agent (MUA): E-Mail client - Thuderbird, Seamonkey, MUTT, etc - Links/Info

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Linux e-Mail Clients:

Linux GUI/X-Window mail clients:

LDAP Enabled:

LDAP enabled email clients have the ability to seamlessly connect to a network accessible "address book" of people's names, email addresses and other information. This allows one to search and select an email address as the recipient from the corporate server. LDAP servers provide this standards based capability. Most email clients which support LDAP can also support a local client based address book as well.

Thunderbird: brought to you by the Mozilla foundation, the creators of the Firefox web browser.
Ubuntu install: sudo apt-get install thunderbird
Also see:
Seamonkey: brought to you by the Mozilla foundation, the creators of the Firefox web browser. Seamonkey is an all-in-one internet application suite: Web-browser, advanced e-mail, newsgroup and feed client, IRC chat, and HTML editing.
Also see:
Balsa: - Gnome project - POP3, IMAP, multi-threading, reads GnomeCard address book, LDAP and render HTML mail. This client (balsa 1.1.1) is actually LDAP enabled but in a clutsy way, it dumps the entire LDAP directory into the address book.
Ubuntu install: sudo apt-get install balsa
Also see:
  • Sylpheed - (LDAP support must be added with source code patch)

Note that Thunderbird and Seamonkey have HTML email rendering built-in. Seamonkey is an integrated application with a built-in browser for integrated browsing of emailed links. Seamonkey integrates the Firefox browser, Thunderbird email client, a web development application, news feed reader and IRC client all provided in one application by the Mozilla foundation.

  • XPine - mail client
  • TradeClient - support for SMTP, POP3, Full IMAP, LDAP (DOES NOT!!! I tried the LDAP support and failed), spell checking.

Web Based e-Mail Clients's:
  • SquirrelMail - PHP4 mail client
  • Zimbra - Includes AJAX based web mail and calendering, antispan, search, backup, archiving, .... Built on top of Tomcat, MySQL, JAVA, Apache, Lucene, ClamAV, SpamAssassin, OpenLDAP and Postfix technologies. [flash pressentation] It rocks.
  • IMP - Provides webmail access to IMAP and POP3 accounts. (Written in PHP)
  • Calacode.com: @Mail - Web and wireless. PERL (source). POP3, IMAP, Calendar, to do lists,... - Commercial product

Console/Text Based e-Mail Clients's:
  • Mutt (Favorite console e-mail app for many of my friends) Supports LDAP.
  • Pine - Program for Internet News & Email
  • Elm - Electronic Mail for UNIX

Commercial e-Mail Clients's:

Support Applications:



Fetchmail is used to download mail using POP or IMAP and route it through port 25, the MTA/SMTP port on your system. It will then be handled by sendmail (or whatever SMTP agent you are using) for the given user account. Typically the mail ends up in the /var/spool/mail/user-name/ directory.


Configuration / Use:

  • Create file: $HOME/.fetchmailrc
        poll mail.your-isp.com protocol POP3 user username password your-password is local-account-login-name
        chmod 600 $HOME/.fetchmailrc
  • Either start or make sure your SMTP daemon is running on port 25. Sendmail example: /etc/rc.d/init.d/sendmail start
  • fetchmail -v --keep -a >> $HOME/fetchmail
    • -v: verbose
    • --keep: keep on server. (don't delete)
    • -a: Retrieve all files.
    • -d interval: Run as a daemon and poll every "interval"
    • -p: protocol
      • POP3
      • APOP
      • IMAP
      • IMAP-LOGIN
      • ETRN


Email Acronyms:

AFAIC As Far As I'm Concerned
AFAIK As Far As I Know
AFK Away From Keyboard
AKA Also Known As
BBS Bulletin Boeard System. The Usenet collections of newsgroups is the worlds largest distributed BBS. Private systems often require membership.
BFD Big F**ing Deal
BFN Bye for now.
BTW By The Way
CIAO Goodby in Italian. Promounced Chow.
EOM End of message. Often put at the end of the Subject heading to let the person know that the whole message was in the Subject and there is no need to open the email.
FAQ Frequently asked questions. Often a list of questions and answers which are most common of first time users of visitors.
Flame/Flame War posting of messages in a hostile manner.
FUBAR F**ed Up Beyond All Recognition
IMHO In my humble opinion
L8R Later
LOL Laugh Out Loud
N/A Not Applicable
OEM Original Equipment Manufacturer
PC Politically Correct
RBTL Read Between The Lines
ROTFLMAO Rolling On The Floor Laughing My Ass Off
ROTFL Rolling On The Floor Laughing
R&R Rest & Relaxation
RTFM Read the F**n Manual
TIA Thanks In Advance
WAG Wild Ass Guess
WYSIWYG What You See Is What You Get
XOXOXO Hugs and Kisses



Email Encryption:

Heavy encryption methods are used to encrypt a decryption key. A lighter encryption method is then used to encrypt the email. ( i.e. SHA, Blowfish, Unix crypt, MD5,...) It is the encryption of the decryption key which is often refered to as that having 128 bit level of encryption with the methods below.

The support of encryption with MIME (Multipurpose Internet Mail Extensions) which allows encryption to be integrated with email clients. Without this support, decryption will be a manual process.

Encryption Protocols:

  • MOSS: (MIME Object Security Services) is an email encryption protocol in accordance with MIME.
  • PGP: (Prety Good Privacy) is considered the de facto standard for email encryption on the internet. PGP performs encryption, decryption, signing, verification, and key management.
  • PGP/MIME: This integrates MIME and PGP so that the email client can handle encryption/decryption of the email rather than the user handling the encrypted email as an attachment which is encrypted/decrypted manually.
    • Message Format: PGP binary
    • Certificate Format: PGP binary
    • Symetric encryption algorithm: TripleDES (DES EDE3 Eccentric CFB)
    • Signature algorithm: ElGamal with DSS
    • Hash algorithm: SHA-1
    • MIME encapsulation of signed data: multipart/signed with ASCII
    • MIME encapsulation of encrypted data: multipart/encrypted
  • S/MIME: Developed by RSA and thus uses RSA encryprion. Multiple versions 2/3. Not designed to be interoperable with with OpenPGP.
    • Message Format: CMS (Cryptographic Message Syntax) (RFC 3369)
    • Certificate Format: X.509v3 (RFC 2632)
    • Symetric encryption algorithm: TripleDES (DES EDE3 CBC)
    • Signature algorithm: Diffie-Hellman (X9.42) with DSS (RFC 2631)
    • Hash algorithm: SHA-1
    • MIME encapsulation of signed data: multipart/signed or CMS
    • MIME encapsulation of encrypted data: application/pkcs7-mime

For more information see the Internet Mail Consortium


  • RFC 2557 - MIME E-mail Encapsulation of Aggregate Documents, such as HTML (MHTML)
  • RFC 2634 - S/MIME V3 - Enhanced Security Services for S/MIME
  • RFC 2633 - S/MIME V3 - Message Specification
  • RFC 2632 - S/MIME V3 - Certificate Handling
  • RFC 2586 - Audio/L16 MIME content type
  • RFC 2426 - vCard MIME Directory Profile
  • RFC 2231 - MIME Parameter Value and Encoded Word Extensions
  • RFC 2156 - Mail headers - Mapping between X.400 and RFC 822/MIME
  • RFC 1327 - Mail headers - Mapping between X.400 and RFC 822/MIME
  • RFC 1148 - Mail headers - Mapping between X.400(1988) / ISO 10021 and RFC 822
  • RFC 1138 - Mail headers
  • RFC 1123 - Mail headers
  • RFC 822 - Mail headers - This explains what e-mail headers look like



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