Your own mailing system (most easiest to use with zero configuration) for the entire world (available in almost all the languages of the world) which will include Mail User Agent and Mail Transfer Agent thus trying to give maximum speed while transferrin
Elmo is a feature-rich console mail client for UN*X power users. It integrates functionality commonly realised by separate pieces of software in other mailers.
Email2Fax and Fax2Email gateway for Asterisk, that could be integrated to your own existing Asterisk.
Microsoft Exchange compatible server and client libraries
OpenChange is a portable Open Source implementation of Microsoft Exchange server and Exchange protocols. It provides a complete solution to interoperate with Microsoft Outlook clients or Microsoft Exchange servers. OpenChange client-side library is used in existing messaging clients and is the solution in new projects to communicate natively with Microsoft Exchange and Exchange-compatible servers. OpenChange server is a transparent Microsoft Exchange server replacement using native Exchange protocols and does not require any plugin installation in Outlook.