rspamd: enable support for greylisting

The threshold is set to match the one for sending mail to the Spam
mailbox.

Author: thestinger

rspamd/local.d/actions.conf

@@ -3,5 +3,5 @@ quarantine = {
   milter = "quarantine";
 }
 
+greylist = 6;
 reject = null;
-greylist = null;