Recently, came across an issue where the outbound SMTP mail where getting blocked. When checked the server and firewall settings everything looked very normal. Finally, the issue was with McAfee. McAfee access protection has a feature to block the out going e-mails. Hence, below listed steps where performed to fix the issue.
1. Click Start, Programs, McAfee, VirusScan Console.
2. Right-click Access Protection and select Properties.
3. Click the Access Protection tab.
4. Under Categories on the left, select Anti-virus Standard Protection.
5. In the right pane, select Prevent mass mailing worms from sending mail.
6. Click on , block tab relevant to “Prevent mass mailing worms from sending mail to remove tick mark.
6. In the Processes to exclude section, type the process name, then click OK to close the Rule details window.
7. Click Apply then close the Access Protection Properties window.