mailutils: support invalid senders

* Workaround for the lack of an e-mail address validation. In case of
  an unsuccessful attempt to send an e-mail due to invalid address,
  admin is notified via e-mail. (closes #2256)

* Documentation of the allow_recurse parameter of the send_email
  function.

Signed-off-by: Daniel Lovasko <daniel.lovasko@cern.ch>