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There is a problem with the proxy server's security certificate. 
The name on the security certificate is invalid or does not match the name of the target site mail.mydomain.com.
Outlook is unable to connect to the proxy server. (Error Code 0)

This just started happening this morning out of the blue but only to ONE user. Everyone else is fine. I tried this problematic user on different computers, same thing. The user can get into OWA just fine, it's only Outlook being problematic. And again, the biggest thing is that it only affects ONE user. I found a bunch of generic solutions about actual certificate issues but none of that applies.

yagmoth555
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prl77
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  • Does the computer query mail.mydomain.com and get mail.mydomain.com certificate ? Hold Left-CTRL and ricght click over the outlook tray icon and select to test the connection. I suspect a bad autodiscover's entry, or something similar. – yagmoth555 Feb 22 '17 at 17:18
  • The computer gets the correct and valid cert in a browser when accessing OWA from the same computer. Outlook doesn't actually open so there is no "tray icon" to click on. Autodiscover would be a global issue since that is hosted on the server, not just with one user. – prl77 Feb 22 '17 at 18:32
  • Yes, but the certificate is pushed by the server, maybe tomorrow it will be a global problem. No, for outlook it can be a different certificate than OWA, try to go there; https://www.server_name.com/rpc and validate the certificate please – yagmoth555 Feb 22 '17 at 18:54
  • Yes, the same valid cert comes in at the RPC address. – prl77 Feb 22 '17 at 19:37
  • As it's valid, does the user got a proxy pushed for him ? outlook use IE setting to connect to get the certificate. – yagmoth555 Feb 22 '17 at 19:55
  • There are no proxies configured anywhere. I double checked because the error talks about that but there aren't any. – prl77 Feb 22 '17 at 19:59
  • What is bugging me is your error code too, **0**. Not listed there; https://support.microsoft.com/en-ca/help/923575/error-message-when-outlook-tries-to-connect-to-a-server-by-using-an-rpc-connection-or-an-https-connection-there-is-a-problem-with-the-proxy-server-s-security-certificate, I'am starting to think it might take less time to re-create another user account for him – yagmoth555 Feb 22 '17 at 20:01
  • As you suspected, the problem is getting bigger. I created some new accounts and they are failing the same way. – prl77 Feb 22 '17 at 20:05
  • As mentioned in this article: – prl77 Feb 22 '17 at 20:11
  • https://community.spiceworks.com/topic/257450-outlook-is-unable-to-connect-to-the-proxy-server – prl77 Feb 22 '17 at 20:11
  • The Microsoft Exchange System Attendant is not running. It's not even there! In the list of service, there are a dozen Exchange services but no System Attendant. – prl77 Feb 22 '17 at 20:12
  • Let us [continue this discussion in chat](http://chat.stackexchange.com/rooms/54086/discussion-between-prl77-and-yagmoth555). – prl77 Feb 22 '17 at 20:15

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