Last week I was installing and configuring MS Exchange in my SharePoint development farm so I can test email notifications in a Visual Studio workflow. I started scouring DNS and did find that I had forgotten to add the MX record for my Exchange server. That still didn’t fix the problem – but was still necessary. The rest of the day was spent verifying that SMTP was configured properly and no other routing issues were found. I finally started looking over other possibilities on the server and found the culprit – McAfee Antivirus. In the Virus Scan Console you need to go to open the Access Protection properties. In there you’ll see a setting that is titled “Prevent mass mailing worms from sending email”. You can edit that setting and add an exclusion for SMTP which resolved the problem. I am enclosing links to articles that provide a step-by-step walkthrough of the topics I mentioned in this post.