Milter-greylist

> > and I could optimize out one member of the struct sync (s_peer).
> > It is not needed since each peer has his own private sync list.
> 
> Not sure I grasped this one. Could you elaborate?
> 

 I suggest to do:
grep s_peer *.[ch]
and you find no more than the three references for it:
1. The declaration in sync.h
2. Its usage in sync_sender() where sync->s_peer is always equal to peer,
3.  since  the value assignment of s_peer in sync_queue() is coming from the
sync list owning peer value.

So in sync_sender() sync->s_peer can be replaced via peer.
No more use of the s_peer, no need then for its assignment,
than no need for its declaration.