Milter-greylist

> i found by chance a strange phenomenon in my logfile: after three "rcpt to"
> to existing(!) addresses, which are marked with
> 
> "milter=greylist, action=rcpt, reject=451 4.7.1 Greylisting in action, please
> come back later"
> 
> as they are not yet known by milter-greylist, sendmail will respond with
> "Possible SMTP RCPT flood, throttling" if throtteling is enabled with
> "define(`confBAD_RCPT_THROTTLE',`3')dnl"
> 
> Somehow, greylisting seems to be evaluated as "bad recipient" by
> confBAD_RCPT_THROTTLE, which seems to be wrong in this case.
> 

Probable explanation is: for the sendmail to decide what recipient is valid or not,
it takes into account the return from the milters as well: 
2xx code means valid recipient, 4xx and 5xx are invalid.

My theory could be verified via reading the source code of sendmail.