Symptoms:

• We will not be able to receive emails from internet and a genuine senders will get the following NDR;
• #550 5.7.1 :67.215.65.132:Client host 131.107.10.4 UnknownDNSName; Mail from IP banned. To request removal from this list please forward this message to delist.forefront@messaging.microsoft.com
• You may get the following Event id as well in the Application log;

Log Name: Application
Source: Microsoft Forefront Protection
Date: 9/7/2010 3:18:07 PM
Event ID: 2098
Task Category: General
Level: Error
Keywords: Classic
User: N/A
Computer: EMAIL.hsh1.com
Description:
The DNS Block list lookup domain blocklist.messaging.microsoft.com could not be contacted. This will prevent DNS Block list lookups. Please verify your network connectivity.

Cause:

FSE  connection filtering  DNSBL checking.

Fix:

• Go for Rollup 1 or disable FSE connection filtering;

• Description of Hotfix Rollup 1 for Microsoft Forefront Protection for Exchange
http://support.microsoft.com/kb/2181692/en-us