Milter-greylist

I has setuped milter-greylist on my server and it works like a charm 
except for some users, they are complaining that they can't send 
emails to our server, instead they are getting rejecting emails 
which is bad since they are using important servers and I dont wanna 
blacklist them and I do not wanna manipulate the blacklist manually.

And some users are getting a very delaying on the email deliverance, 
even up to 12 hours which is not good since we has set in the 
greylist.conf a maximum time on 3h for delivery and normal delat on 
5 minutes.

Can anyone please let me know this please.

1. What can be the reason to both things above ?

2. How can I trace emails on the database to see whats going wrong ?

3. How can I empty the blacklist and/or whitelist manually ?

Any other tip concering these things would be very much 
appreciated :)

Sincerely
/Roberth