Psmail is a set of Perl scripts that, when used together, are similar to using an email program. Messages are stored one-per-file in specified directories, and utilities are given to create template new messages (writing headers, etc), reply, read, and list messages with relevant info (i.e. #lines, subject, who from, etc.). As each file is a separate message, you can also write complex commandlines to conditionally handle files. Psmail doesn't handle MIME or multiple POP accounts (yet).
Saké Mail is a very high performance Web-based email system, designed for scaling to large numbers of concurrent users. It takes advantage of the Java Servlet API's memory-resident architecture and multi-threading HTTP response model. It avoids file system accesses through memory caching at every possible point, and never needs to touch the file system at runtime at all if coupled with an IMAP server. It supports POP3, IMAP, and SMTP, including authenticated SMTP. It also provides an extremely sophisticated message rendering system that can easily handle even complex HTML encoded messages with attachments that have been forwarded multiple times.
SendEmail is a lightweight, completely command line-based SMTP email delivery program. If you have the need to send email from a command prompt this tool is perfect. It was designed to be used in bash scripts, Perl programs, and Web sites, but it is also quite useful in many other contexts. It is written in Perl, and is unique in that it requires no special modules, and supports any platform that runs Perl. It has a straightforward interface, making it very easy to use.
Shohei is a multi-language, multi-function, multi-server, multi-user, multi-context, multi-media Web-based mail client, news client, calendar, and note server. Users can be in any number of domains on any number of servers. Web-based administration with different admins for different domains. Features include an online calendar, notes, users' personal Web pages, a news client, and is MIME aware.
OpenGroupware.org is a set of applications for contact, appointment, project, and content management. It is comparable to Exchange and SharePoint. It is accessible using Web interfaces or using local clients via WebDAV, GroupDAV, and CalDAV. Custom applications can be developed using the XML-RPC API. OpenGroupware.org runs on almost every GNU/Linux system.
SmartSign is a set of modules which allow integration of smartcard technology into an OpenCA based Public Key Infrastructure in order to provide smartcard-based digital signature and local authentication security services. It allows direct signing of e-mail and e-news from within Netscape using smartcards and supports signing of generic files from command line. The package includes a PAM module too, which allows system administrators to integrate smartcard-based authentication for local users. A modified version of the OpenSSH client allows secure authentication to a remote server. A couple of command line tools allow signing and verifying generic files from the shell. Finally, a command line interactive shell supports all operations on the card, and can be used to write scripts that automate particular tasks on the card. Currently only Schlumberger Cyberflex Access 16K is supported.