outlook Code 0x800ccc78

How to Fix Outlook Error 0x800ccc78

Diagnostic Procedures

  • 1 Understand the cause of Outlook Error 0x800ccc78
  • 2 Enable Outgoing SMTP Server Authentication
  • 3 Correct your Display and Sender Email Address
  • 4 Clear Cached Credentials on Windows
  • 5 Configure macOS accounts to require SMTP authentication
  • 6 Summary checklist for error 0x800ccc78

How to Fix Outlook Error 0x800ccc78

When trying to send an email in Microsoft Outlook, you may experience the following error: “Sending reported error (0x800ccc78): Cannot send the message. Verify the e-mail address in your account properties. The server responded: 550 Access Denied” (or similar “Authentication Required” codes).

This error indicates that your outgoing mail server (SMTP) has rejected the communication because it did not receive valid user authentication credentials or because the sender’s address was formatted incorrectly.

1. What Causes Outlook Error 0x800ccc78?

This validation error typically stems from:

  • Disabled Outgoing Server Authentication: By default, Outlook may attempt to send emails without logging into the SMTP server, which secure mail servers reject.
  • Sender Address Mismatch: The email address defined in your account properties does not match the actual email address linked to your login credentials, triggering anti-spoofing blocks (Error 550).
  • Corrupted Credentials in Credential Manager: Saved authentication tokens are mismatched or expired.

2. Enable Outgoing SMTP Server Authentication

The most frequent fix is ensuring Outlook logs into the SMTP server using the credentials associated with your incoming mail account.

Step 1: Open More Settings

  1. Press the Windows Key + R, type control, and press Enter to open the Control Panel.
  2. Select Mail (Microsoft Outlook) → Click Email Accounts….
  3. Select your email address and click Change….
  4. Click the More Settings… button in the lower-right corner.

Step 2: Configure the Outgoing Server

  1. In the Mail Setup window, switch to the Outgoing Server tab.
  2. Check the box for “My outgoing server (SMTP) requires authentication”.
  3. Select “Use same settings as my incoming mail server”.
  4. Click OK, then click Next to run account test diagnostics. Close the wizard when it completes.

3. Correct your Display and Sender Email Address

If your email address is misspelled, containing special characters or invalid domain spacing, the SMTP server will reject the transmission with a 550 error code.

  1. Navigate to Control Panel → Mail → Email Accounts… → Select your account → Click Change….
  2. Examine the E-mail Address field. Ensure it does not contain typos, extra spaces, or mismatched uppercase letters.
  3. If using an alias, ensure the mailbox hosting the SMTP server has explicit authorization to send emails using that alias.

4. Clear Cached Credentials on Windows

Stale entries in the Windows Credential Manager will cause Outlook to send invalid login packets to the SMTP server.

Step 1: Terminate Outlook Processes

Close Outlook completely and run the following command in Command Prompt to kill any lingering services:

taskkill /f /im outlook.exe

Step 2: Delete Credentials via Cmdkey

Open a Command Prompt and search for cached Outlook credentials:

cmdkey /list | findstr "MicrosoftOffice"

To delete the corrupted credentials, open the Credential Manager (control keymgr.dll), navigate to Windows Credentials, look for entries starting with MicrosoftOffice16_Data:orgId: or generic mail server hostnames, and click Remove.

5. Configure macOS Accounts to Require SMTP Authentication

On macOS, Outlook handles outgoing server authentication preferences within its Accounts interface.

Step 1: Clear macOS Keychain Credentials

Open Terminal and delete cached Office and Outlook keychain passwords:

security delete-generic-password -s "Microsoft Outlook" 2>/dev/null
security delete-generic-password -l "MicrosoftOffice15_2identity:ADAL" 2>/dev/null

Step 2: Adjust Account Settings in Outlook for Mac

  1. Open Microsoft Outlook.
  2. Go to Tools → Accounts from the menu bar.
  3. Select the target account and click More Options… beneath the Outgoing server configuration.
  4. In the Authentication dropdown menu, select Use Incoming Server Info instead of “None”.
  5. Click OK and close the Accounts settings window.

6. Summary Diagnostics Checklist

Diagnostics StepAction / PathExpected Outcome
SMTP AuthenticationMail Options -> Outgoing Server -> “Use same settings as incoming”Prevents the server from rejecting anonymous send attempts.
Verify Sender AddressChange Account Settings -> E-mail Address fieldEliminates formatting typos causing Server 550 blocks.
Flush Windows CredentialsCommand Prompt -> control keymgr.dllCleans outdated authentication tokens causing password loops.
Flush Mac KeychainTerminal -> security delete-generic-passwordForces macOS Outlook to request a fresh token from SMTP.
Sync TestClick Send/Receive in the Outlook navigation barConfirms the SMTP error 0x800ccc78 is resolved.