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The following is the environment:

One Exchange 2013 and one 2016 server. The 2016 server is in hybrid mode. None of the the client access URLs point to the 2016 server FQDN. Autodiscover.domain.com (split DNS) points to autodiscover.outlook.com as all mailboxes (except journaling and some test mailboxes) are in Office 365.

The official cert is installed on Exchange.

Some Outlook clients get a certificate warning and it shows that they are trying to connect to the local Exchange 2016 server's FQDN - which gives the error of course because this name is not on the certificate.

Any idea how Outlook keeps connecting to this while none of the CAS URLs point to this FQDN and while autodiscover points to Exchange Online?

Thanks

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The problem has been resolved. This was the issue: Autodiscover was changed to point to Exchange Online while some users still had some manually added shared mailboxes in their profile of mailboxes that were never moved to Exchange Online.

So even though the Autodiscover response did not contain any reference to on-premises servers, the Outlook cache was still trying to connect to an on-premises server.

Removing the "orphaned" mailboxes from the profiles solved the problem.

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