One configuration setup that I have run into a a number of times is the lack of outbound Spam/Anti-Virus filtering. The organization in question has their inbound mail queue running through their Anti-Spam/AV filter, but they do not have their outbound mail going through it. When asked about it, typically the response has been, “Never thought about it. We are really only concerned about inbound spam and viruses.”
Here is my typical response:
When a corporate mailbox is compromised and is used in a spam campaign, how will you know? When the user of the compromised account complains to the helpdesk because of the glut of bounced messages in his inbox? By that time, possibly your only static public IP has been listed on a number of Real-time Blackhole Lists (RBL).
Filtering outbound messages for Spam/Malware helps you detect internal compromised accounts/computers faster than waiting for the user or RBL to alert you. Just be sure to tweak alerts to get the right kind of alerts when they are needed, so that you don’t fall into alert fatigue.
-Josh
Defensive Depth 