about milter

sendmail plugin architecture
library for plugins, server needs milter support, too
sendmail only, not for postfix etc ...
generic handlers work on state transitions in SMTP
connect, HELO, mail from, rcpt to, headers, end of headers
body, end of body, end of message, abort
listen on local domain socket or network socket
IPv4, IPv6
allows for one milter server to work with multiple SMTP servers