I wrote a
"shim" program that accepts incoming SMTP connection and
 passes them on to the real MTA when it gets a valid RCPT.  It was
 designed to turn away bogus RCPTs very very cheaply (and it works
 spectacularly well for that[%]), but it also does banner delay, DATA
 delay, and early talker rejection.  (Because it connects through to a
 real MTA in real time, it avoids the worst of the hairy issues that
 plague MTAs, like queueing and retries.)
 I wrote this for work, but they've given me approval to release it (I
 asked preemptively); I can put it up for FTP if there's any interest. 
 I would, for one, be quite interested in your implementation. I have
 greet_pause implemented in sendmail, and some fairly draconian custom
 rules, but it needs all the help it can get. 
 
<AOL>Me too!</AOL>
Peace...  Sridhar