Why my Mail Enable sending junk emails

Hi Team,
 
My Mail Enable is sending out junk emails. At the moment many seem to originate from "postmaster@domain.com" which is a domain on this system, but is not a mail domain.
 
At other times, the emails are "from" and "to" completely external domains. Its almost like I am an open relay, but I am not...
I have checked the SMTP configuration and it seems OK. Only authenticated users can send out.
I have checked the logs and I cannot understand how these emails are getting in and out of the system.
 
I need some help to stop this from happening...
 
Regards