PP sending email to unknown users

Hi All

This last week I've been experiencing a strange issue when sending out messages and comments to a certain client.

After sending the message, I'll get a MAILER-DAEMON bounce back of the message stating that a handful of email addresses aren't deliverable. There are 7 or 8 addresses, non of which are users of the system, or people I know at all for that matter. I'm wondering how many other non-users are getting emails as well.

Could my install have become compromized? Any idea what the issue could be?

Thanks,
Bryan