While greylisting has severe drawbacks, it's reasonably common.
This patch extends the default hook for greylisting, using "greylistd".
Completing authentication or passing SPF will make it skip the greylist
check, to avoid some of the most common issues with it.
It is can be convenient for hooks to indicate that an error is
permanent; for example if the anti-virus found something.
This patch makes it so that if the hook exits with code 20, then it's
considered permanent. Otherwise it is considered transient, to help
prevent accidental errors cause final delivery issues.
This patch implements a post-DATA hook, which is run after receiving the
data but before sending a reply.
It can be used to implement content filtering when receiving email, for
example for passing the email through an anti-spam or an anti-virus.
