Coming straight from my mailbox:
Electus Matari has placed your corporation on its valid targets list (please see http://www.electusmatari.com/standings.cgi for the full list), due to your crimes in Republic space (namely, piracy).
The -5 standing means that our combat pilots are authorized to fire on ships of your organization without warning. It does not usually mean they will hunt you down.
If you need more information, or feel there has been a mistake, please feel free to contact me at any time.