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Summary: Outlook Express, the predecessor to Windows Mail, was available until Windows Vista. With no Microsoft user support, users must handle errors independently. This article addresses the 0x800CCC91 POP3 INVALID USERNAME error and provides potential causes and solutions, including checking username accuracy and modifying antivirus settings. For more complex issues, Kernel for Outlook Express, a DBX recovery tool, is recommended to recover damaged DBX files while preserving email formatting and properties. It’s a reliable solution for Outlook Express users.

Outlook Express served as the precursor to Windows Mail and was available until Windows Vista. While some users may liken it to a lightweight version of Outlook, they are distinct applications. If you still use Windows Vista, you might encounter Outlook Express. However, be aware that Microsoft no longer offers user support for this software, leaving you to resolve any issues independently. This article addresses a specific error that can arise when using Outlook Express with a POP account.

0x800CCC91 POP3 INVALID USERNAME – Invalid username or username not found

If you’ve already linked the account with the same username in Outlook Express, and the error persists, it’s crucial to investigate potential causes and identify the most effective methods for resolving it.


  • You have entered wrong/invalid user name.
  • Your anti-virus has been set to automatically scan emails during send/receive process.
  • Your mailbox might have been corrupted.


You can resolve the “0x800CCC91 POP3 INVALID USERNAME – Invalid username or username not found” error message using the following methods:

  • Check if the user name you have entered is correct or not. If the user name is wrong, retype the user name. Check your account login details through webmail. If you have configured POP3 account (Gmail, yahoo mail..) then you can request for recovery of user name and password from the email provider itself.
  • If the anti-virus installed on your computer system has been configured to automatically scan incoming/outgoing emails then this problem is very common. Reconfigure the anti-virus settings to do not automatically scan emails.
  • Uninstall the anti-virus of your system and then check if the error still appears.
  • Take backups of your DBX databases and reconfigure the Outlook Express account profile.

Kernel for Outlook Express is a DBX recovery software designed to help you retrieve emails from damaged DBX files by expertly repairing them. During the recovery process, this software preserves email formatting and properties, eliminating the need for further modifications in the recovered emails. With Kernel for Outlook Express, you can seamlessly repair DBX files created in Outlook Express versions 4.0 and above. Visit this link to learn more about how to repair outlook express database.


Traditional solutions to resolve this error don’t offer a foolproof fix. When you’re certain that your Outlook Express file is corrupted, it’s time to turn to Kernel for Outlook Express. This software will efficiently recover your entire mailbox content, offering you the flexibility to save it in your preferred format. Plus, you can seamlessly install and use it on Windows Vista to repair your DBX files.

Kernel for Outlook Express