Milter-greylist

I've heard users describe instances where mail is delayed for about a
day.  This time, I had the time to dig up (and scrub) the logs.

I'm running milter-greylist 3.0 and sendmail 8.13.8 on Debian.

The important part of the log to note is:

Mar 6 16:17:37 milter-greylist from sender to userb delays 00:17:12
Mar 6 16:17:37 sm-mta from sender via relay 123.45.67.89
Mar 7 20:42:18 milter-greylist addr 123.45.67.89 from sender to userb
               autowhitelisted for 504:00:00

sender uses only one mail relay in all transactions.
sender has no problems sending the same message to usera.
userb got sender's message 27 hours after usera got it.
... why?

Thanks,
Adam