Applies to: Outlook for Mac, Outlook 2016 for Mac, POP & IMAP Mailboxes
How to fix certificate warnings in Outlook 2016 for MacOS (POP/IMAP)
Certificate warnings in Outlook typically occur when the mail server’s SSL certificate is not trusted or does not match the server’s hostname. This can happen due to various reasons such as an expired certificate, a misconfigured mail server certificate, or a mismatch between the server hostname and the certificate’s common name.
When Outlook performs an Auto-discover during start up of Microsoft Outlook may receive a warning message that resembles the following:
“A secure connection cannot be established with the server because its intermediate or root certificate cannot be found. Do you want to continue? If you continue, the information that you view and send will be encrypted, but will not be secure.“
The easiest way to bypass this issue, is to click Continue to accept the warning. However, the warning will reappear the next time that Auto-discover runs or outlook is restarted.
Cause
This issue often occurs in Outlook 2016 for Mac version 15.9 and later versions when Outlook performs an Auto discover operation and tries to connect to a service endpoint whose expected name is not present on the server’s Secure Sockets Layer (SSL) certificate.
Resolution
To resolve this issue, use one of the following methods:
Method 1
Reissue a mail server certificate that includes the domain name as the Subject Alternative Name.
This enables you to resolve the issue for all Outlook for Mac clients without having to trust the certificate from each client individually.
You may need to contact your email provider to apply this method.
Method 2
This solution enables you to resolve the issue quickly for individual Mac clients without having to reissue the certificate.
Trust the certificate on the Mac client.
When you receive the certificate warning message, click Show Certificate.
Once the certificate is shown onscreen you should then select the “Always trust <https://www.domain.com> when connecting to <https://mailserver.com>” check box, and then click continue.
Restart Outlook and verify the issue has been resolved, if the issue is still present you can follow the advanced troubleshooting steps below or contact your email provider for further guidance.
Further Troubleshooting
Step 1) Understand the Issue
Certificate warnings in Outlook typically occur when the mail server’s SSL certificate is not trusted or does not match the server’s hostname. This can happen due to various reasons such as an expired certificate, a misconfigured certificate, or a mismatch between the server hostname and the certificate’s common name.
Step 2) Verify Server Settings
Ensure that you have the correct incoming and outgoing mail server settings configured in Outlook. Double-check the server hostname, port numbers, and SSL settings.
Step 3) Check Certificate Validity
Verify the SSL certificate installed on your mail server. You can do this by accessing your mail server’s web interface or contacting your email hosting provider. Make sure the certificate is valid and has not expired.
Step 4) Update Outlook
Ensure that you are using the latest version of Outlook 2016 for Mac OSX. Microsoft frequently releases updates to fix bugs and address security issues. Updating Outlook may resolve certificate warnings.
Step 5) Add Certificate to Keychain Access
If the SSL certificate is valid but not trusted by your system, you may need to manually add it to your Keychain Access. Follow these steps:
- Open Keychain Access from Applications > Utilities.
- Click on “Certificates” in the Category section on the left sidebar.
- Drag and drop the SSL certificate file into the Keychain Access window.
- Double-click the imported certificate and change the “Trust” dropdown to “Always Trust.”
- Close Keychain Access.
Step 6) Restart Outlook
After adding the certificate to Keychain Access, restart Outlook to apply the changes. This should prevent certificate warnings from appearing.
Step 7) Disable SSL/TLS Certificate Validation
If you’re still experiencing certificate warnings, you can try disabling SSL/TLS certificate validation in Outlook:
This step should only be considered as a temporary workaround and not a permanent solution, as it compromises security.
If this fixes the issue you need to contact your email provider to resolve the problem properly to avoid further security issues.
- Go to Outlook > Preferences.
- Click on “Accounts.”
- Select your email account from the list.
- Under “Account Information,” click on “Advanced.”
- Uncheck the boxes for SSL and TLS certificate validation.
- Close the Preferences window.
Step 8) Contact Your Email Provider
If none of the above steps resolve the issue, contact your email hosting provider for assistance. They can verify the server configuration, SSL certificate, and provide further guidance on resolving certificate warnings in Outlook.
By following these steps, you should be able to troubleshoot and fix certificate warnings in Outlook 2016 for Mac OSX when using POP or IMAP configurations.
