How to Fix Outlook Error 0x800CCC0F Lost Connection on Send

Outlook error 0x800CCC0F appears when you try to send an email and the connection to your mail server is lost. This error means Outlook started talking to the outgoing SMTP server but the server ended the session before Outlook could finish sending. The problem is almost always on your side — a misconfigured outgoing server port, an incorrect authentication setting, or a security program blocking the connection. This article explains why error 0x800CCC0F happens and gives you step-by-step fixes to restore outgoing mail.

Why Outlook Error 0x800CCC0F Occurs When Sending Email

Error 0x800CCC0F is a socket-level error. It means Outlook opened a TCP connection to the SMTP server but the server closed the connection before Outlook could send the email. This is not a server outage — the server actively rejected or dropped the connection after it was established.

The most common root causes are:

• Wrong SMTP port or encryption setting. Many email providers no longer support port 25 or require SSL on port 465. Using the wrong combination causes the server to drop the connection immediately.
• Authentication failure. The outgoing server requires authentication, but Outlook is set to “Anonymous” or “Use same settings as my incoming mail server” is unchecked. The server sees an unauthenticated request and terminates the session.
• Security software interference. Antivirus, firewall, or VPN software intercepts the SMTP traffic. If the software times out or modifies the packet, the server drops the connection.
• Network proxy or ISP restrictions. Some corporate networks or ISPs block port 25 or 587 to prevent spam. Outlook cannot complete the SMTP handshake and reports error 0x800CCC0F.

Steps to Fix Outlook Error 0x800CCC0F

Perform the following fixes in order. Test sending an email after each step to see if the error is resolved.

Fix 1: Verify SMTP Port and Encryption Settings

1. Open Outlook and go to File > Account Settings > Account Settings
   In the Email tab, select your email account and click Change.
2. Click More Settings and switch to the Advanced tab
   Find the Outgoing server (SMTP) section. Set the port number to 587. For the encryption method, select TLS or STARTTLS from the dropdown. If your provider requires SSL, set port 465 and encryption to SSL. Common provider settings are listed in the table below.
3. Click OK and then Next, then Finish
   Send a test email. If the error persists, proceed to Fix 2.

Fix 2: Enable SMTP Authentication

1. Go back to File > Account Settings > Account Settings > Change > More Settings
   Select the Outgoing Server tab.
2. Check “My outgoing server (SMTP) requires authentication”
   Below that, select Use same settings as my incoming mail server. This forces Outlook to send the same username and password used for incoming POP3 or IMAP.
3. Click OK, then Next, then Finish
   Send a test email. If the error continues, move to Fix 3.

Fix 3: Temporarily Disable Antivirus Email Scanning

1. Open your antivirus or internet security suite
   Look for settings related to email protection, SMTP scanning, or mail shield. Common products: Norton, McAfee, Bitdefender, Avast, Kaspersky.
2. Disable the email scanning feature temporarily
   Do not disable the entire antivirus — only the email or SMTP scanning component. The exact option is often called “Scan outgoing email” or “Mail protection.”
3. Send a test email from Outlook
   If the email sends successfully, add Outlook.exe to the antivirus exclusion list. In most security suites, go to Settings > Exclusions > Add process and browse to C:\Program Files\Microsoft Office\root\Office16\OUTLOOK.EXE. Re-enable email scanning.

Fix 4: Disable Proxy or VPN Temporarily

1. Disconnect from your VPN client
   Right-click the VPN icon in the system tray and select Disconnect.
2. Open Windows Settings > Network & Internet > Proxy
   Ensure “Use a proxy server” is turned Off. If your organization requires a proxy, contact your IT department for the correct SMTP exception.
3. Send a test email
   If the email sends, the VPN or proxy was blocking SMTP. Reconnect the VPN and configure split tunneling to exclude Outlook traffic, or use the corporate SMTP server address.

Fix 5: Create a New Outlook Profile

1. Close Outlook and open Control Panel
   In Windows 11, search “Control Panel” from the Start menu. Click Mail (Microsoft Outlook).
2. Click Show Profiles and then Add
   Give the new profile a name like “Outlook New.” Enter your email address and password. Outlook will auto-configure the settings.
3. Set the new profile as default
   Under “When starting Microsoft Outlook,” select Always use this profile and choose the new profile from the dropdown. Click OK.
4. Open Outlook and send a test email
   If the error is gone, your old profile had corrupted settings. You can delete the old profile later.

If Outlook Still Shows Error 0x800CCC0F After the Main Fixes

Outlook error 0x800CCC0F on one specific email account only

This usually means the account’s SMTP settings are unique. Double-check the provider’s documentation for the exact server name, port, and encryption. For example, Gmail requires smtp.gmail.com on port 587 with TLS. Outlook.com requires smtp-mail.outlook.com on port 587 with STARTTLS. If you use two-factor authentication, generate an app password and use it in Outlook instead of your regular password.

Error appears only when sending to a specific domain

This is rare but can happen if the recipient’s mail server has a greylisting policy or a strict SPF/DKIM check. The sending SMTP server may interpret the delay as a lost connection. Wait 15 minutes and try again. If the error persists, contact your email provider’s support to check if they throttle connections to that domain.

Error appears after a Windows update

Some Windows updates change network stack behavior or reset Winsock. Open Command Prompt as administrator and run netsh winsock reset. Restart your computer. Then run netsh int ip reset. Restart again. This restores the TCP/IP stack to default and often resolves connection drop issues.

Common SMTP Ports and Encryption Settings for Major Providers

Provider	SMTP Server	Port	Encryption
Outlook.com / Hotmail	smtp-mail.outlook.com	587	STARTTLS
Gmail	smtp.gmail.com	587	TLS
Yahoo Mail	smtp.mail.yahoo.com	587	TLS
Office 365 / Exchange Online	smtp.office365.com	587	STARTTLS
iCloud Mail	smtp.mail.me.com	587	TLS

You can now fix error 0x800CCC0F by checking the SMTP port and encryption, enabling authentication, and disabling security software interference. After applying the correct fix, test sending an email to confirm the connection is stable. If you manage multiple accounts, use the table above to enter the exact settings for each provider. As an advanced step, enable Outlook logging (File > Options > Advanced > Enable troubleshooting logging) to capture the exact server response when the error occurs — this data helps your IT team or provider diagnose persistent issues.