0x800CCC18OfficeHigh
Error 0x800CCC18 — SSL Security Error | Microsoft Office Fix
Microsoft 365Office 2021Office 2019Office 2016
What Does 0x800CCC18 Mean?
Outlook encountered an SSL/TLS security error when connecting to the mail server. The secure connection could not be established.
Real-World Causes
- 1Mail server SSL certificate expired or invalid
- 2SSL/TLS version incompatibility
- 3Antivirus intercepting and breaking SSL connections
- 4Incorrect encryption settings in Outlook account configuration
Symptoms
- ⚠Cannot connect to mail server via SSL/TLS
- ⚠Security certificate warning messages
- ⚠Send/Receive fails with security error
- ⚠Outlook prompts about untrusted certificate
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DIY Fix
Beginner-friendly steps you can try at home
- 1Verify SSL/TLS encryption settings in account configuration
- 2Try changing from SSL to TLS or vice versa
- 3Disable antivirus email scanning/SSL inspection temporarily
- 4Check if the mail server SSL certificate has expired
Advanced Fix
For experienced users and IT professionals
- 1Update Windows root certificates via Windows Update
- 2Clear the SSL state: Internet Options > Content > Clear SSL State
- 3Import the mail server's certificate into Windows certificate store
- 4Check TLS version compatibility with the server
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Frequently Asked Questions
Should I disable SSL to fix this?
No. Disabling SSL means your email credentials and messages are sent unencrypted. Instead, fix the underlying SSL issue — update certificates, adjust TLS settings, or check antivirus interference.
Can antivirus cause SSL errors in Outlook?
Yes. Many antivirus programs intercept SSL/TLS connections to scan email content. This can cause certificate trust errors. Try disabling the antivirus email scanning feature specifically.
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About the Author
Windows Troubleshooting Team
Verified against official Microsoft documentation and real-world diagnostic data. Error behavior confirmed across Windows 10, Windows 11, and Windows Server.