Distributed Checksum Clearinghouse (DCC) is a system of clients and servers that collect and count checksums related to mail messages. The counts can be used by SMTP servers and mail user agents to detect and reject bulk mail. DCC servers can exchange common checksums. The checksums include values that are "fuzzy", or constant across common variations in bulk messages.
Zimbra is a next-generation enterprise messaging and collaboration system. It features an Ajax Web client with conversation views, fast email search, Web service mash-ups, shared calendars, document authoring, and IM. The server also syncs to MS Outlook (via MAPI), the Apple desktop (via iSync, iCal, and CalDAV), POP/IMAP clients, and mobile devices (BlackBerry, smart phones, HTML Web browsers). A version of the Web client, Zimbra Desktop, also works offline with any POP/IMAP server. Administrators can control live backup/restores of individual or groups of mailboxes, hierarchical storage (HSM) and clustering, LDAP directory configuration, Web services integration with existing enterprise applications, and integrated anti-spam and anti-virus.
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.
phlyMail is a groupware, webmailer, and PIM. It offers unlimited nested groups, unlimited POP3/IMAP profiles, quotas, an integrated address book, a calendar with multiple reminders and event repetitions, a file manager with an integrated WebDAV server, the ability to send faxes, SMS support (which lets you send SMS, get reminded of events via SMS, and get email notification via SMS), a bookmark manager, themes, and multilingual support. It uses either sendmail or SMTP to send email, and can handle MIME and SSL connections. Support for IDN (Internationalized Domain Names) is included. Through its modular design, it's easy to add completely new features as modules, which can be plugged into the system.
Dovecot is an IMAP server whose major goals are security and extreme reliability. It uses index files to optimally store the mailbox state, which makes it very fast even with huge mailboxes. Indexes won't prevent external mailbox updates, so Dovecot is still fully compatible with standard Maildir and mbox formats. There's also a fully featured POP3 server included.
AMaViS (A Mail Virus Scanner) scans e-mail attachments for viruses using third-party virus scanners available for UNIX environments. It resides on a UNIX (Linux) machine and looks through the attached files arriving via e-mail, generates reports when a virus is found and sets the delivery on hold.
amavisd-new is a high-performance and reliable interface between MTAs and one or more content checkers, including virus scanners, and/or the Mail::SpamAssassin Perl module. It talks to an MTA via (E)SMTP or LMTP, or by using helper programs. No timing gaps exist in the design, which could cause a mail loss. It is normally positioned at or near a central mailer, not necessarily where the user's mailboxes and final delivery takes place.
MIMEDefang is a flexible MIME email scanner designed to do all kinds of email processing, including anti-virus scanning, anti-spam scanning, replacing parts of messages with URLs, adding boilerplate disclaimers, and so on. It can alter or delete various parts of a MIME message according to a Perl-based policy filter. It can also bounce messages with unacceptable attachments and modify SMTP response codes on the fly. It works with the Sendmail 8.11 and newer "Milter" API, which makes it more flexible and efficient than procmail-based approaches.