Error 0x800CCC90 - What is it?
Error 0x800CCC90 is a typical Outlook error. This error indicates that Outlook has failed to connect to the mail server that handles and manages the incoming and outgoing emails. The error message is displayed in the following format:This error limits your ability to send and receive emails on Outlook. If you largely communicate at the workplace using your Outlook account, this error may slow down your work productivity and performance dramatically. Therefore it is advisable to fix it immediately.“There was a problem logging onto your mail server. Your User Name was rejected.”
Solution
Error Causes
The error 0x800CCC90 is triggered several reasons including but not limited to:
- Authentication issues
- Failure of POP 3 Server
- PST File Corruption
- Registry issues- Invalid Entries
- Viral infection
Further Information and Manual Repair
To resolve error 0x800CCC90, try the methods given below:Method 1 - Adjust the Authentication Settings
To resolve the error 0x800CCC90, adjust the authentication settings. Here’s what you need to do:- First, go to the ‘Tools’ tab and select the ‘Accounts’ option.
- Once you select the account option, an internet accounts box will appear on the computer screen.
- Simply double click on the account property box after that go to the server tab and click on it.
- Check the box that says ‘My Server Requires Authentication’.
- Click on apply to accept changes tab to save changes and close the window.
- To activate changes made, restart Outlook and then try sending emails again.

In the dialog, type in netplwiz and press ENTER.
You will find yourself in the User Accounts window, inside deselect Users must enter a user name and password to use this computer. Press OK
Sign in the window will pop up where you will have to type in your password and confirm it in order to start the feature.
After typing in your password and confirming it, press OK. That's it, next time you wake your computer from hibernation or turn it ON, you will automatically be greeted with a desktop instead of a Windows login screen. 
