“0x800CCC78” Unknown Sender in Outlook

When attempting to send an email using Microsoft Outlook configured with an IMAP or POP account, the transmission may fail instantly with an error in the Send/Receive Progress window:

Task 'user@domain.com - Sending' reported error (0x800CCC78):
'Cannot send the message. Verify the e-mail address in your account properties.
The server responded: 550 5.7.1 / 530 5.7.0 Authentication Required'

Quick Answer

Outlook error 0x800CCC78 occurs when your outgoing mail server (SMTP) requires authentication, but Outlook is configured to send messages anonymously, or when the “E-mail Address” field in your account settings is blank or malformed. To fix it, enable “My outgoing server (SMTP) requires authentication” in Outlook’s Advanced Account Settings.

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What the Signal Means

The hex code 0x800CCC78 is an internal Microsoft Outlook error translated from Winsock/SMTP status errors. It means: “Unable to send because the sender address was rejected by the server, or the client failed to establish sender authorization.”

When Outlook prepares to send a message, it executes a specific sequence:

1. EHLO [ClientName]
2. AUTH LOGIN (Submits your credentials)
3. MAIL FROM: <user@domain.com>
4. RCPT TO: <recipient@domain.com>

Error 0x800CCC78 triggers during step 2 or 3. If Outlook skips step 2, the server refuses to recognize the sender in step 3. Alternatively, if the email address listed in Outlook’s profile contains an accidental trailing space or a syntax error, the server’s parser rejects the address outright.

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Technical Causes

Outlook Error 0x800CCC78
│
├── Did you recently set up or reconfigure this account?
│   ├── YES ──► "SMTP Authentication" checkbox is turned off.
│   └── NO  ──► Malformed address string or antivirus proxy interception.
│
├── Does the error mention "550" or "Relaying Denied"?
│   ├── YES ──► The server thinks you are an unauthorized third party. Turn on SMTP Auth.
│   └── NO  ──► Malformed email field (missing '@' or illegal characters).

1. Disabled Outgoing SMTP Authentication: By default in older setup flows, Outlook assumes the outgoing server does not require a password. Modern mail servers reject all unauthenticated outbound connection attempts.
2. Malformed “Email Address” or “Reply-To” Field: Typing user@domain .com (accidental space), using missing angle brackets, or leaving the user email address field completely blank causes an immediate rejection during the MAIL FROM: command.
3. ISP Port 25 Throttling and Interception: If your outgoing account is set to port 25, your ISP may intercept the traffic and route it through a transparent proxy. The proxy rejects your credentials because it does not recognize your identity.
4. Third-Party Antivirus Mail Scanners: Security applications that scan outbound mail (such as Avast, AVG, or Bitdefender) act as a local “man-in-the-middle.” If the security software cannot negotiate authentication with the remote host, Outlook reports error 0x800CCC78.

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Progressive Diagnostic Steps

Step 1: Enable SMTP Authentication (Primary Fix)

In 90% of occurrences, enabling outgoing authentication resolves the problem immediately:

1. Open Outlook and go to File > Account Settings > Account Settings.
2. Select the problematic email account and click Change.
3. In the setup window, locate and click the More Settings button in the bottom right corner.
4. Click the Outgoing Server tab.
5. Check the box: My outgoing server (SMTP) requires authentication.
6. Select the radio button: Use same settings as my incoming mail server.

!Outlook Outgoing Server Settings Dialog

1. Click OK, then click Next, and finish the test.

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Step 2: Validate Account Identity Fields

A subtle typo in your profile information will trigger an immediate rejection:

1. In the same Account Settings > Change Account window, review the User Information section:
   • Your Name: Ensure this is not completely empty.
   • Email Address: Verify there are no leading or trailing spaces, misplaced commas, or illegal punctuation characters (e.g., name@domain..com or name@domain,com).
2. Click More Settings > General.
3. Inspect the Reply-To Email field. If this field contains an invalid address, clearing it completely or fixing the typo will resolve the error.

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Step 3: Align SMTP Ports and Encryption Protocols

If authentication is enabled but the server continues to reject the sender, ensure your outgoing port and encryption protocols match:

1. Go to File > Account Settings > Account Settings > Change > More Settings > Advanced.
2. Under Outgoing server (SMTP):
   • If using port 587, set the encryption dropdown to STARTTLS (or TLS).
   • If using port 465, set the encryption dropdown to SSL/TLS.
   • Avoid using port 25, which is widely blocked or restricted by modern internet providers.

For more details on resolving port-encryption mismatches, see How to Fix Gmail Port 587 vs. 465 Timeouts.

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Step 4: Disable Local Outgoing Mail Inspection

If the problem persists despite matching settings:

1. Open your third-party antivirus program’s control center.
2. Find the settings for Email Protection or Mail Shield.
3. Temporarily disable Scan outbound emails (SMTP).
4. Restart Outlook and attempt to send the queued message from your Outbox.

If the email goes through, update your antivirus software or configure it to bypass encrypted mail ports (465 and 587). This allows Outlook to talk directly to your mail server without local network interference.