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Getting (Proxy server's security certificate) errors 18 and 10

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Last Modified: 2015-10-12
Hello experts,

We have installed a new Exchange 2013 on a new Win 2012 server. The older exchange is 2010 on a Win 2008R2 server.

We plan to uninstall old Exchange 2010 later, right now we have all mailboxes migrated to the new exchange.

Now both mobile app access and owa are working well. We have the issue only with desktop Outlook. For few users only it's working well, for almost 90% of users they are receiving the famous message:

There is a problem with the proxy server's security certificate. The security certificate is not from a trusted certifying authority. Outlook is unable to connect to the proxy server mail.ourdomain.com. (Error code 18).

BTW, we don't have any signed certificate, but only using self-signed ones which are setup automatically upon installation of Exchange.

Any help solving this?
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