This will block connections on receiving a 'RCPT TO:' request. Alternatively you can configure the
dispatcher (using dispatcher.cnf) as follows:
!...rest of normal options...
The latter case, the incoming connection will be rejected at the banner.
This is how the SORBS SMTP proxy server works by default. It's disadvantage
is some clients (incoming connections from servers are considered clients to)
ignore the 554 banner as a PERMANENT fail and keep trying to resent until the
message times out.