Some time over the last month, an issue has arisen with our exchange 2016 server Autodiscovery.

Attempting to configure an outlook 2016 client I could not connect to the exchange 2016 server. The client is being configured on the internal LAN. I have attempted with another Outlook 2016 client and had the same issue.

This is Exchange 2016 with the latest CU, updates, and extended Protection Enabled, on prem, NOT a hybrid.

The Outlook client is 2016 with the latest patches on Win 10. Adobe is the only 3rd party app with hooks in Outlook. AV is Defender.

DNS is correct for both internal and external access. Pings from internal and external return the proper ip addresses.When I attempt to browse to the autodiscovery.xml via a browser, it comes up with a http 500 error. It does not prompt me for a password. This happens on WIN 10 client and on the exchange server as well.

When I run the Microsoft Remote Connectivity Analyzer It fails at the autodiscover POST with an http 404 and 500 “The Microsoft Connectivity Analyzer failed to obtain an Autodiscover XML response”. The certificate tests as a valid certificate.

Internally running the Test E-Mail AutoConfiguration from Outlook, I get that the Autodiscover is found through SCP then continuously fails with 500 and 404 errors. The failed erros are 0x800C820f, 0x80070057, and 0x8004010f (SRV Record Check). I have also run this externally with the same result.

I have reset the VDir on the exchange server but it has not resoloved the issue. I have checked the authentication for the VDir and it is set as recommended with permissions according to Ali Tajran’s guides.

Browsing the page from IIS gives the same 500 error in a browser.

The exchange server has EP enabled as well as the URL Rewrite module. Server has been locked down to use TLS 1.2 and the best practices template from IISCrypto 3.3 applied. I have tried disabling EP temporarily, but this did not resolve the issue.

Everyone that is currently connected to exchange is not having an issue. Anyone attempting to connect to exchange cannot connect due to the autodiscover issue.

Then only things I have done in the past few months was install the March 2024 update, then enabled extended protection. The certificate was rekeyed after the march updates. Then after the mess of the March patch I applied the hotfix. Since I have added one user that was from a fresh install of office 2016.

Followed various guides from Ali Trajan and others I have googled. Been spinning on this as I have tried just about everything I can think of.