We maintain a large list of IP addresses that are known senders of spam and viruses. When someone connects to the Messaging Gateway from one of these IP addresses, we are set to reject them by default. You can get a report of the IP addresses we have rejected by going to the Message Audit Logs on the Status page and run a report. To find these emails you will want to place a period in the mandatory filter value and set the optional filter to verdict, and the value to look for ip addresses in the global bad sender list.