Outlook error 0x800CCC90 shows up when Outlook cannot authenticate with the mail server, usually right after sending or receiving mail. It almost always points to a login or server-connection problem rather than a damaged mailbox, and clearing it takes just a few targeted checks.
Key Takeaways
- Outlook error 0x800CCC90 is a server authentication failure, most often caused by an incorrect password, an expired app password, or a blocked port.
- Re-entering credentials and refreshing the account connection fixes the issue in most cases.
- Antivirus email scanning and outdated server settings are common secondary triggers.
- Modern Outlook accounts using OAuth (Microsoft 365, Outlook.com) rarely need manual server changes.
- Older POP or IMAP accounts may need updated port and encryption settings to resolve the error for good.
How to Fix Outlook Error 0x800CCC90 Step by Step
Work through these steps in order. Most users find the fix in the first two or three steps, so there is no need to jump straight to the more technical settings unless the basic checks do not resolve it.
Update Login Credentials and Account Settings
- Step 1: Open Outlook and go to File, then Account Settings, then Account Settings again.
- Step 2: Select the affected email account and click Change.
- Step 3: Re-enter the account password exactly as it appears with the email provider. A single typo or an old saved password is the single most common cause of this error.
- Step 4: If the account uses two-factor authentication, generate a fresh app-specific password from the provider’s security settings and paste it into the password field instead of the regular login password.
- Step 5: Click Next, let Outlook test the account settings, then click Finish and restart Outlook.
Tip: If Outlook keeps prompting for a password even after entering the correct one, clear saved credentials first through Windows Credential Manager, then re-enter them in Outlook. Stale cached logins are a frequent reason the same wrong password keeps getting resent to the server.
Check Server Port and Encryption Settings
- Step 6: Return to Account Settings and select the account, then click Change followed by More Settings.
- Step 7: Open the Advanced tab and confirm the incoming and outgoing server ports match what the email provider recommends (commonly 993 for IMAP and 587 for SMTP, both with TLS encryption).
- Step 8: If the ports look outdated, such as using port 25 without encryption, update them to the current recommended values and set the encryption type to TLS or SSL as specified by the provider.
- Step 9: Click OK, then Next, then Finish, and send a test message to confirm the connection goes through cleanly.
Temporarily Disable Antivirus Email Scanning
- Step 10: Open the antivirus program’s settings and locate the email or Outlook protection module.
- Step 11: Turn off email scanning temporarily and try sending or receiving mail again in Outlook.
- Step 12: If the error disappears, update the antivirus software to its latest version before turning scanning back on, since outdated scan modules are known to intercept and break the authentication handshake.
Common Problems and Fixes
- Error returns immediately after re-entering the password: The account likely still has cached credentials elsewhere. Clear entries related to the account in Windows Credential Manager, then restart Outlook before logging in again.
- Test account settings fail during Step 5: Double-check that the incoming and outgoing server addresses have no extra spaces or typos, since even a single stray character will trigger this authentication error.
- Error only happens when sending, not receiving: This usually means the outgoing (SMTP) server requires authentication that has not been enabled. In More Settings, open the Outgoing Server tab and confirm ‘My outgoing server (SMTP) requires authentication’ is checked.
- Error appears only on one device: The issue is local to that Outlook profile. Try creating a new Outlook profile through the Control Panel’s Mail settings and adding the account fresh.
- Corporate or work account still fails after all steps: Contact the IT administrator, since some organizations block specific ports or require a VPN connection for mail authentication to succeed.
Frequently Asked Questions
What does Outlook error 0x800CCC90 actually mean?
It means Outlook attempted to connect to the mail server but the server rejected the login credentials or the connection request, most often due to a wrong password or an authentication mismatch.
Is Outlook error 0x800CCC90 caused by a corrupted mailbox?
No. This particular error is tied to server authentication and connection settings, not to file corruption. Mailbox repair tools are not needed to clear it.
Why does the error appear only after changing a password?
Outlook often keeps the old password cached, so it keeps sending outdated credentials to the server until the saved login is cleared and re-entered.
Does two-factor authentication cause this error?
It can, if Outlook is still using the regular account password instead of a generated app-specific password. Accounts with two-factor authentication typically require an app password for third-party mail clients like Outlook.
Can antivirus software really trigger this error?
Yes. Some antivirus programs scan outgoing and incoming mail traffic and can interfere with the authentication handshake, especially if the software is out of date.
Does this error mean the email account has been hacked?
Not necessarily. While a changed or reset password can cause it, the error itself is simply Outlook failing to authenticate and does not confirm unauthorized account access.
Why does the error only happen when sending mail?
This usually points to the outgoing server not being set to require authentication in Outlook’s account settings, even though the provider requires it.
Will creating a new Outlook profile fix the error?
In many cases, yes, particularly when the error only affects one device or one Outlook profile while other devices connect to the same account without issue.
Does this fix apply to both Microsoft 365 and Outlook.com accounts?
Yes, the same troubleshooting steps apply, though Microsoft 365 and Outlook.com accounts using modern authentication rarely need manual port or encryption changes.
How long does it typically take to resolve this error?
Most cases clear up within a few minutes once the correct password or app password is entered and the account connection is refreshed.
