Fix Bug when trying to send a mail after the SMTPMailSender threw

exeption or all recipients were removed/empty

Added a abort() function to cancel the current process.

In case the SMTPMailSender throws an exception which is catched by another script or the recipientlist was completely empty, no further mails could be sent because the last command was not aborted/the status was not read out.
It may be a good idea to use a different exception type than "SystemException"