1. Mozilla Thunderbird
Mozilla Thunderbird is a free, open source, cross-platform e-mail and news client developed by the Mozilla Foundation. Features like quick search, saved search folders , advanced message filtering, message grouping, and labels help manage and find messages. Just like Firefox, the tons of extensions and themes for this client makes it very secure and flexible to enhance your productivity.
Most secure e-mail and news client: Thunderbird provides enterprise and government-grade security features such as SSL/TLS connections to IMAP and SMTP servers. It also offers native support for S/MIME secure email (digital signing and message encryption using certificates). Any of these security features can take advantage of smartcards with the installation of additional extensions.Other security features can be added through extensions. For instance, Enigmail offers PGP signing, encryption, and decryption.
Optional security protections also include disabling loading of remote images within messages, enabling only specific media types (sanitizer), and disabling JavaScript. The French military uses Thunderbird and contributes to its security features, which are claimed to match the requirements for NATO’s closed messaging system.
2. Sylpheed
Sylpheed is a free, GTK+-based, open source email and news client. It is very light lightweight. Sylpheed runs on Windows, Mac OS X and Unix-like systems such as Linux, and BSD. It has following features:
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 e-mail 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 strong 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 e-mail clients. Storing several ten thousands of messages doesn’t affect the stability. Even if the application 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 powerful 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 powerful 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.
3. Claws Mail
Claws Mail is a free, GTK+-based, open source email and news client. It offers easy configuration and an abundance of features. It stores mail in the MH mailbox format and also the Mbox mailbox format via a plugin. Claws Mail runs on both Windows and Unix-like systems such as Linux, BSD, Solaris, and Mac OS X. Claws Mail provides many features, among them are:
- Search and filtering
- Security (GPG, SSL, anti-phishing)
- Import/export from standard formats
- External editor
- Templates
- Foldable quotes
- Per-folder preferences
- Face, X-Face support
- Customisable toolbars
- Themes support
- Plugins
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