Fast Bayesian spam filter along lines suggested by Paul Graham
Remote anti spam utility
Anti Spam SMTP Gateway
Open source DMARC implementation
Protects you from spam mails and virus mails effectively with milter
The free cross-platform antivirus software tool-kit
Outgoing email rate limiter
Converts quoted-printable, UTF-8, UTF-16BE, UTF16LE to 8-bit.
Tools to convert Z mark-up to HTML or text.
Blocks spam without blocking any legitimate messages
OpenSource high performance mail policy daemon for Postfi
CRM114 milter for sendmail
QSF milter for sendmail