Balsa is a lightweight email client for GNOME

GNOME

GNOME is a desktop environment and graphical user interface that runs on top of a computer operating system. It is composed entirely of free and open source software...

.

Balsa has a graphical front end, support for MIME

MIME

Multipurpose Internet Mail Extensions is an Internet standard that extends the format of email to support:* Text in character sets other than ASCII* Non-text attachments* Message bodies with multiple parts...

 attachments coming and going, directly supports POP3

Post Office Protocol

In computing, the Post Office Protocol is an application-layer Internet standard protocol used by local e-mail clients to retrieve e-mail from a remote server over a TCP/IP connection. POP and IMAP are the two most prevalent Internet standard protocols for e-mail retrieval. Virtually all modern...

 and IMAP

Internet Message Access Protocol

Internet message access protocol is one of the two most prevalent Internet standard protocols for e-mail retrieval, the other being the Post Office Protocol...

 protocols. It has a spell checker and direct support for PGP

Pretty Good Privacy

Pretty Good Privacy is a data encryption and decryption computer program that provides cryptographic privacy and authentication for data communication. PGP is often used for signing, encrypting and decrypting texts, E-mails, files, directories and whole disk partitions to increase the security...

 and GPG

GNU Privacy Guard

GNU Privacy Guard is a GPL Licensed alternative to the PGP suite of cryptographic software. GnuPG is compliant with RFC 4880, which is the current IETF standards track specification of OpenPGP...

 for encryption. It has some basic filtering capabilities, and natively supports several email storage protocols. It also has some internationalization support, including Japanese fonts.

It builds on top of these other open source packages: GNOME

GNOME

GNOME is a desktop environment and graphical user interface that runs on top of a computer operating system. It is composed entirely of free and open source software...

, libtool

Libtool

GNU Libtool is a GNU programming tool from the GNU build system used for creating portable compiled libraries. To quote the introduction in the :Libtool is typically used with Autoconf and Automake, two other tools of the GNU build system....

, libESMTP, aspell

GNU Aspell

GNU Aspell, usually called just Aspell, is a free software spell checker designed to replace Ispell. It is the standard spell checker for the GNU software system. It also compiles for other Unix-like operating systems and Windows. The main program is licensed under the GNU Lesser General Public...

, and gmime. It also can optionally use libgtkhtml for HTML rendering, libkrb5 for GSS-API

Generic Security Services Application Program Interface

The Generic Security Services Application Program Interface is an application programming interface for programs to access security services....

, and openldap

OpenLDAP

OpenLDAP Software is a free, open source implementation of the Lightweight Directory Access Protocol developed by the OpenLDAP Project. It is released under its own BSD-style license called the OpenLDAP Public License. LDAP is a platform-independent protocol. Several common Linux distributions...

 for LDAP

Lightweight Directory Access Protocol

The Lightweight Directory Access Protocol is an application protocol for accessing and maintaining distributed directory information services over an Internet Protocol network...

 functionality. It can optionally be configured to use gpg-error and gpgme libraries.