Starting about 3 weeks ago, I started receiving many e-mails each day that say "Mail delivery failed: returning message to sender", or "Returned mail: see transcript for details" or "Returned mail: User unknown" or "Undeliverable" followed by some bogus message (i.e., stapler rendering, instituitional V, etc.). The e-mails are being addressed to some random collection of characters @ our website address (i.e., firstname.lastname@example.org).
I don't know if these are all just incoming spam or some virus attempt, OR if there is a computer on our network that has a worm that is actually sending email to all of these addresses and having it rejected - and then the rejections come to me as the default address for our e-mail.
I have we scan for viruses weekly and have not found any. And both our incoming and outgoing e-mails are scanned as well.
How do I determine where these are originating from? And how can I get them to stop?