mailutils: support invalid senders
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) (PR #2385)

* Documentation of the forward_failures_to_admin parameter of the
  send_email function.

Signed-off-by: Daniel Lovasko <9052ae34ac0fed81689ad21063de293132fb8181@cern.ch>
Reviewed-by: Samuele Kaplun <2e218f779e12714c314fad8ebf6b3041daa21337@cern.ch>
Reviewed-by: Tibor Simko <c7116e45300b163846dc01f3ed02116e3f8ef7a5@cern.ch>

inveniosoftware/invenio,tiborsimko/invenio,tiborsimko/invenio,inveniosoftware/invenio