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 forward_failures_to_admin parameter of the
  send_email function.

Signed-off-by: Daniel Lovasko <daniel.lovasko@cern.ch>
Reviewed-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>