To resolve this problem, disable the antivirus e-mail protection utility as well as the Windows Firewall, to disable the Windows Firewall, follow these steps:

1. Open the Control Panel (Start > Control Panel)
2. Open Network Connection
3. Right click on the internet connection you’re using (LAN, or Wireless, or GreenNet Dial-up)
4. From the menu select ‘Properties’
5. Go to the ‘Security’ tab, and switch off the Firewall.

Then, if you haven’t already done so you need to manually add the incoming POP server name (pop.gn.apc.org) and the outgoing SMTP server name (smtp.gn.apc.org) in the E-mail Accounts dialog box in Outlook.

To manually add the POP server setting, follow these steps:

1. Start Outlook.
2. On the Tools menu, click E-mail Accounts.
3. Click View or change existing e-mail accounts, and then click Next.
4. In the E-mail Accounts dialog box, click Add.
5. Click POP3, and then click Next.
6. Type the correct information for your POP3 mail server, and then click Next.
7. Click Finish.

Source: Microsoft Knowledge Base Article – 816866