How to Fix Outlook Error 0x800ccc67

When attempting to send emails in Microsoft Outlook, you may receive the following error message: “Sending reported error (0x800ccc67): An unknown error has occurred. Please save any existing work and restart the program.”

This error typically indicates that your Simple Mail Transfer Protocol (SMTP) server rejected the connection or was suddenly cut off mid-transmission. It prevents outgoing emails while leaving incoming mail unaffected.

1. What Causes SMTP Error 0x800ccc67?

This SMTP transmission error is triggered by the following issues:

• Incorrect SMTP Configuration: Mismatch between the outgoing port and the required encryption protocol (SSL vs. TLS).
• Port Blocking by ISP: Many Internet Service Providers (ISPs) block Port 25 by default to combat spam.
• Antivirus/Firewall Interference: Security programs scanning outgoing emails can drop the connection before the mail server can respond.
• Network Packet Fragmentation (MTU): The Maximum Transmission Unit size is too large for the active network route, causing larger emails to fail to send.

2. Adjust Outgoing SMTP Port and Encryption Settings

Using incorrect ports or matching the wrong encryption type to the port will cause the SMTP server to terminate the handshake.

Step 1: Open Account settings

1. In Outlook, click File → Account Settings → Account Settings.
2. Select your email account from the list and click Change….
3. Click More Settings → Go to the Advanced tab.

Step 2: Configure Ports and Security Protocol

Apply one of the standard secure configurations:

• Secure SMTP Option A (Recommended):
  • Outgoing server (SMTP): 587
  • Use the following type of encrypted connection: TLS or STARTTLS
• Secure SMTP Option B:
  • Outgoing server (SMTP): 465
  • Use the following type of encrypted connection: SSL or SSL/TLS

Click OK, click Next, and allow Outlook to perform its account sync test.

3. Temporarily Disable Antivirus and Email Scanning

Antivirus applications scan outgoing mail by inserting themselves as local proxies. If their mail scanning engine hangs, the connection times out and outputs error 0x800ccc67.

Disable your security suite’s “Email Scanning” or “SMTP Protection” feature. If you use Windows Defender, you can test if the firewall is blocking Outlook with this PowerShell command:

Get-NetFirewallRule -DisplayName "Microsoft Outlook" | Select-Object Name, Enabled, Direction, Action

4. Verify Network Port Availability

Ensure that your firewall or ISP is not actively blocking outbound connections on SMTP ports.

Test-NetConnection -ComputerName smtp.office365.com -Port 587

nc -zv smtp.office365.com 587

5. Adjust MTU Settings to Prevent Packet Loss

If you can connect to the SMTP server but emails with attachments fail with 0x800ccc67, packet fragmentation may be occurring. Lowering the MTU (Maximum Transmission Unit) limits the packet size.

netsh interface ipv4 show subinterfaces

Step 2: Lower the MTU Value

If your active connection is named “Wi-Fi” or “Ethernet”, run the corresponding command to lower the MTU limit to 1400 bytes:

netsh interface ipv4 set subinterface "Wi-Fi" mtu=1400 store=persistent
netsh interface ipv4 set subinterface "Ethernet" mtu=1400 store=persistent

Step 1: Force Close Outlook and Reset Network Interfaces

Run the following Terminal script to clear orphaned app processes and reset macOS interface caches:

killall "Microsoft Outlook" 2>/dev/null
killall "Microsoft Outlook Finder Integration" 2>/dev/null
sudo ipconfig set action change

7. Summary Diagnostics Checklist