Gemini

Gemini is an advanced, multi-platform, standards compliant, messaging and calendaring client with powerful features for managing mail and news from multiple sources. Gemini provides flexible filtering of messages and protection from junk mail, viruses, and common breaches of netiquette. It has an attractive interface which is easy to use, yet hides nothing from the more advanced user. Additional features include mailing lists as pseudo-newsgroups, message templates, auto-replies, support for multiple identities, HTML, IMAP, SSL, editing of stored messages, spell checking, syntax coloring, and encryption/signing via integration with GnuPG.