Got an No-Reply Email Virus from my own Domain?

Hey Spiceheads, office was recently hit with cryptolocker on one of our users, I have since then re-instated everything to working condition, however since this attack, I have noticed some users getting an email with a .zip file with the Subject line of "Your Xerox Workcentre Copy Is Ready" the weird part is, it seems this email is being sent from a no-reply@mydomain.com  , any ideas? I have been getting a lot of these spammy looking emails in our server lately but they are all sent from what looks like to be legit domains, there fore my spam filters are not picking it up.