NDR: No permission to send to this recipeint.

Problem:  NDR with the following messge

      user1@ohwhere.com on 7/29/2013 4:50 PM

            You do not have permission to send to this recipient.  For assistance, contact your system administrator.

            < smtp01.yourcompany.com #5.7.1 SMTP; 550 5.7.1 :127.0.0.3:Client host 123.123.123.123 blocked using 88.blocklist.zap; Mail from IP banned. To request removal from this list please forward this message to delist.forefront@messaging.microsoft.com>

Solution:

1. Go to ESM>First Administrative Group>Servers
2. Select the server where the mailbox of user resides
3. Go to Protocols>SMTP
4. Right click on Default SMTP Virtual Server, click on Properties.
5. Go to Access Tab>Relay button.
6. Check Allow all computers...