The Meldware Communications Suite (MCS) is a platform-independent groupware package consisting of: Meldware Mail, Meldware Calendar, Meldware Webmail, Meldware Secure Administration System. It provides IMAP, POP, SMTP, iCal, WCAP, and Webmail services.
Mailatt is a portable Unix/Linux script for sending mail with MIME attachments. The Content-Type and Content-Transfer-Encoding are guessed, but can be overruled with commandline options. There is support for HTML mail and non-ASCII character sets.
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.