If an alert message manages to loop back into the ticketing system, refuse
posting to the ticket thread. Technically, the message should be marked as
an auto-response message; however, auto-response messages should usually be
allowed to be appended to the ticket thread.

This patch will check if the From email header cites an email address that
is a system email address (visible in the Emails section of the Admin
Panel). If it is, the email is completely ignored.