Sylpheed 2.5.0 Final is a Email Client product from sylpheed.good-day.net, get 5 Stars SoftSea Rating, Sylpheed is a simple, lightweight but featureful, and easy to use Email client (mailer, MUA). Sylpheed runs on Unix-like systems such as Linux, BSD, and Mac OS X. It also supports Windows. Sylpheed uses GTK+ GUI toolkit. The newest version of Sylpheed works with GTK+ 2.4 or later (2.6 or later is recommended). Sylpheed is a free program distributed under the GNU GPL (the library part is GNU LGPL). You can freely use, modify and redistribute it under the license.
Features:
Simple, beautiful, and well-polished user interface
Comfortable operationality which is built in detail
Immediately available with minimal configuration
Lightweight operation
High reliability
Internationalization and Multilingualization support
High-level Japanese processing
Various protocols support
Security features (GnuPG, POP3/IMAP4/NNTP over SSL/TLSv1)
Powerful filtering
Junk mail control
Flexible cooperation with external commands
Whats New in version:
A crash bug was fixed.
Content-Transfer-Encoding header with comments was supported.
UTF-8 locale strings with suffix .utf8 were supported.
The 8KB restriction of template body size was removed.
Features Detail:

Simple, beautiful, and well-polished user interface
Sylpheed adopts 3-paned display similar to the popular Email clients for Windows such as Outlook Express and Becky. So it is easier to use it on the first time.
Comfortable operationality
The user-interface is constantly improved, and Sylpheed achieves attentive, comfortable operationality.
Especially the operationality of keyboard is taken count of, so you can sequentially read mails efficiently. Of course, you can use easy mouse-only operation.
You can also customize the behavior of mail display in detail.

Well-organized, easy-to-understand configuration
Each configuration item is well-organized, so you can easily configure Sylpheed when you start using it.
The default settings are also cared for, so you only have to configure required Email account settings to use it immediately (other settings can be kept as default).
Lightweight operation
The performance is always cared when implementing Sylpheed because it exerts a powerful influence on the usability. For example, you can read messages in a folder which has 20-30 thousands without stress.
The every portion of Sylpheed is implemented by C language (no interpreter or virtual machine), so there is no sluggishness.
High reliability
The stability and the protection of data is taken into account to the fullest extent because they are the most important factors for Email clients. Storing several ten thousands of messages doesn't affect the stability. Even if the software is forcibly terminated, there is almost no possibility of the lost of mail, or inconsistent state of mailboxes.
Messages are managed with MH format on which a file corresponds to a message, so you can easily backup, migrate, and process them with other applications. There is also less risk of mail loss when a system failure occurs, compared to formats on which a file corresponds to a folder. Messages are instantly moved or deleted with this format when asked to, so users don't have to perform periodic compaction of mailboxes by hand to prevent the growth of mailboxes.

Powerful filtering and search
You can perform various processings for sent/received messages by using the strong filtering feature.
Any headers, message body, size, date, flag status, and external commands can be specified as the conditions of a filter. For actions as conditions match, move, copy, delete from server, setting flags, the execution of external commands, etc. can be performed.
Sylpheed also has strong search function which utilizes this filtering engine. Moreover, you can easily refer to the messages which matches search conditions by saving the result as a search folder. It also has the quick search function which quickly narrows down the message list by flag types and subject strings.

Junk mail control
Sylpheed has a learning-type junk mail control feature which utilizes external commands. You can automatically filter junk mails without setting filter rules by hand, so you don't have to worry about being buried in the flood of junk mail anymore. It also has high flexibility. Other than the bogofilter and bsfilter, which are the default, you can use any programs as the external commands. The standard programs can be easily used from Sylpheed by just installing them.
Internationalization and multilingualization support, and high-level Japanese processing
Sylpheed supports internationalization (i18n) and multilingualization (m17n) (currently translated to 30 languages). The internal editor also implements internationalized line-wrapping.
Sylpheed also supports the high-level Japanese processing, such as the stable character encoding conversion by using the original implementation, code guessing (ISO-2022-JP/EUC-JP/Shift_JIS/UTF-8), and the support for the environment-dependent characters.
Various protocols support
Sylpheed supports not only the major protocols such as POP3, IMAP4rev1 and SMTP, but also NNTP (NetNews). IPv6, which is the next-generation Internet protocol, is also supported by default.
Security features (GnuPG, SSL/TLSv1)
Sylpheed optionally supports the signing and encryption of messages by GnuPG, and the encrypted communication of POP3/IMAP4/NNTP over SSL/TLSv1.
Flexible cooperation with external commands
You can call external commands and cooperate with them in various circumstance. The Action feature can be used as a filter by passing the currently selected messages to external commands. You can use external editors in the message composition. You can also utilize external commands for the filtering on receiving messages.
If the receiving feature of Sylpheed is not enough, you can even use fetchmail and/or procmail, or external commands (such as inc, imget) for receiving. The license of this email software is Freeware, you can free download and free use this Email software.