GnuPG (the GNU Privacy Guard or GPG) is GNU's tool for secure communication and data storage. It can be used to encrypt data and to create digital signatures. It includes an advanced key management facility and is compliant with the proposed OpenPGP Internet standard as described in RFC2440. As such, it is meant to be compatible with PGP from NAI, Inc. Because it does not use any patented algorithms, it can be used without any restrictions.
Courier is a mail transfer agent (MTA) and groupware server. It implements IMAP, ESMTP, POP3, LDAP, SSL, HTTP, iCal. It also provides mailing list management, filtering, maildirs storage, and a webmail interface with calendaring and scheduling.
pam_mapi is a PAM module for authenticating against a Kopano or Zarafa server. It is intended to be used as a bridge between SASL daemons providing SMTP authentication and a Kopano/Zafara user information database.
Thunderbird is a desktop mail client. It's a companion product to Mozilla Firefox. Besides standard email, it supports newsgroups, feed reading, and chat protocols (XMPP, IRC and Twitter), all with multiple identities or accounts. Through Mozilla Lighning or other plugins it's extensible into a full personal information manager. Out of the box it supports SMTP, POP3, IMAP, SSL/TLS, S/MIME, LDAP, RSS/Atom, and searching, filtering or grouping, labels, virtual folders, and comes with a spam filter
Postgrey is a Postfix policy server that implements greylisting. When a request for mail delivery is received by Postfix via SMTP, the triplet CLIENT_IP / SENDER / RECIPIENT is built. If it is the first time that this triplet is seen, or if the triplet was first seen less than 10 minutes ago, then the mail gets rejected with a temporary error. Hopefully, spammers or viruses will not try again later, even though such behavior is required per RFC.
Z-Push is an implementation of the ActiveSync protocol which is used 'over-the-air' for multi platform ActiveSync devices, including Windows Mobile, Android, iPhone, Sony Ericsson and Nokia mobile devices. With Z-Push any groupware can be connected and synced with these devices.
The Zarafa Collaboration Platform is a Microsoft Exchange replacement. Zarafa provides an integration with an existing Linux mail server, native mobile phone support by ActiveSync compatibility and a WebApp with 'Look & Feel' similar to Outlook using Ajax. Including an IMAP and a POP3 gateway as well as an iCal/CalDAV gateway, Zarafa can combine the usability with the stability and the flexibility of a Linux server.
Cascade is a small Python utility to copy emails between IMAP servers.