Quotient is multi-protocol (SMTP, POP, IMAP, SIP, HTTP, Q2Q) server that helps with all your online conversations, be they over email, IRC, IM, mailing lists, or voice over IP. It is written in Python on the Twisted framework, and uses Xapian for search and SpamBayes for spam classification.
BitDefender Mail Protection is a powerful mail filtering solution that provides antivirus and antispam capabilities to Linux mail servers. Both sent and received messages are scanned in real-time, avoiding possible infections and preventing anyone from sending an infected message. It claims a 100% detection rate for all viruses in the wild through its powerful scanning engines, which are certified by prestigious testing labs.
postfix_memcached is a patch to Postfix (2.1 and 2.2) to allow it to use memcache as a map source. The memcache map type can be used to reduce the load on another map type, e.g. pgsql. By placing the memcache map before the pgsql map, a key not found in the memcache will fall through to the database. You'll need to figure out a way to reverse populate the cache from the database yourself. This map type can be seen as an alternative to the proxymap(8) service.
XamimeLT is an email scanner which encapsulates your existing sendmail or Postfix server setup. It scans both incoming and outgoing email. It can scan for email viruses, unwanted file types (eg. EXE, BMP, MPEG) and file names (eg. prettypark.exe), spam, and almost anything you care to search for in an email. It can also be used to scan for text snippets within emails and to insert disclaimers into outbound emails. XamimeLT was formerly known as "Inflex".
mysqmail-postfix-logger is a (very small) daemon that logs all SMTP traffic to an SQL database. It splits the information by domain and user for the From and To fields, making it very easy to count all the SMTP traffic for a given domain name. DTC (Domain Technologie Control) uses it for logging the SMTP traffic.