Certificate Warning in Outlook 2016 Mac

Applies to: Outlook for Mac for Office 365 Outlook 2016 for Mac – Non Office 365 Email Systems Only

Symptoms

Assume that Microsoft Outlook 2016 for Mac performs an Autodiscover operation. For example, Outlook performs an Autodiscover operation during Outlook start or periodically while Outlook runs. In this scenario, you 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.

Outlook for Mac Certificate Error

When you receive this warning message, you can click Continue to accept the warning. However, the warning may reappear the next time that Autodiscover runs.

Cause

This issue occurs in Outlook 2016 for Mac version 15.9 and later versions when Outlook performs an Autodiscover 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 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.

Method 2

Trust the certificate on the Mac client. This solution enables you to resolve the issue quickly for individual Mac clients without having to reissue the certificate.
When you receive the certificate warning message, click Show Certificate.
Select the Always trust <https://www.domain.com> when connecting to check box, and then click Continue.

Always trust the certificate option after clicking Show Certificate

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