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