Though, Outlook Express is fast and simple to use email client developed by Microsoft Corporation, some error messages make it difficult to smoothly work with it. One of the common error messages that flashes on your computer screen while using Outlook Express is:

0x800CCC90 POP3 RESPONSE ERROR – Client response invalid

Preceding error message is generated due to following reasons:

Windows Firewall is turned on due to which Outlook Express is having problem in connecting with mail server.
Your anti-virus has been set to automatically scan emails during send/receive process.

You can resolve the issue by following below given measures:

• Turn Off Firewall: Though, firewall has been embedded in your computer system to protect your computer from unauthorized access, sometimes it creates problem for Outlook Express in connecting with mail server. Therefore, you should turn off firewall for an instance to check if the problem has been occurring due to it or not. Following steps you turn off firewall:

1. Open Control Panel through Windows Start Menu.
2. Double-click Windows Firewall icon. Windows Firewall screen appears.
3. Click Off (not recommended) option.
4. Click OK.

After turning off the firewall, continue using Outlook Express. If the issue is resolved then later on turn on the firewall or keep it off. But, if the problem does not resolve then try next solution.

• Modify Anti-virus Settings: If you have configured your anti-virus to automatically scan all incoming/outgoing emails then sometimes it create problems in processing emails through Outlook Express. Therefore, you should modify your anti-virus settings to do not automatically scan emails. After modifying anti-virus settings, restart the anti-virus.

Above mentioned solutions will help you resolve the 0x800CCC90 POP3 RESPONSE ERROR – Client response invalid error message.

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