The domain was probably whitelisted , either system wide, or one or more users.
We prohibit whitelisting, spam overrides, and ither user riutung capabilities because of the headaches tbey create.
You literally have to check every user account and domain to clear them out. One user had mire than 1,500 routing rules setup, with many forwarding to accounts which no longer exists, and didn't understand why she was receiving her e-mail. Block and disabled whitelisting - server wide.
Phonr: (773) 491-9019
Phone: (224) 444-0169
E-Mail and DNS Security Specialist
Network Security Specialist
Customer Service Portal: https://portal.chicagonettech.com
Security Blog: http://networkbastion.blogspot.com/
Web and E-Mail Hosting, E-Mail Security and Consulting