Milter-greylist

On Tue, Feb 11, 2014 at 03:18:48PM +0000, Emmanuel Dreyfus wrote:
> 
> Sometimes, milter-greylist gets overloaded, and stops answering requests
> in time. sendmail  says it goes "to error state", and I have to restart
> milter-greylist to get it working again.
> 
> Today I looked closly at the problem and found that the frozen 
> milter-greylist had more than 1600 threads.

Is there any hint what these threads are doing?
What says 
pstack PID_OF_MG_PROCESS
?