Hook output is checked to see if it looks like a header, which includes
the possibility of multi-line headers.
This patch extends the tests to include a multi-line header, to prevent
accidental regressions.
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.