Rmilter is used to integrate rspamd and milter compatible MTA,
for example postfix or sendmail. Rmilter can also do other useful stuff:
  Clamav scanning (via unix or tcp socket).
  Rspamd scanning
  Greylisting with redis upstream
  Ratelimit with redis upstream
  Auto-whitelisting (internal and via redis upstream)
  Replies check (whitelisting replies to sent messages)
  Passing messages and/or their headers to beanstalk servers
