question

SusanDodds-3538 avatar image
0 Votes"
SusanDodds-3538 asked EricYin-MSFT edited

Outlook is unable to connect to the proxy server mail.domain.com (Error Code 80000000).

Exchange 2016/2019 coexistence. Certificate being used is a self-signed created on Exchange 2016, installed on 2019.

I've seen the error twice under normal operations and once when testing 2019 by taking 2016 off the network to make sure 2019 would work properly without 2016.

Arbitration mailboxes haven't been moved to 2019.

URL's on 2019 are the same as 2016.

Everything else working correctly besides this occasional error.

I found a couple of mentions of this error from a couple of years ago but none have offered a solution that has helped me.

Thanks

office-exchange-server-deployment
· 1
5 |1600 characters needed characters left characters exceeded

Up to 10 attachments (including images) can be used with a maximum of 3.0 MiB each and 30.0 MiB total.

"URL's on 2019 are the same as 2016."
If you only use one namespace, it should be pointing to 2019. For detailed info, see https://techcommunity.microsoft.com/t5/exchange-team-blog/client-connectivity-in-an-exchange-2016-coexistence-environment/ba-p/603945
Check this link and see if these steps help: https://www.outlookfaq.com/errors/resolve-outlook-is-unable-to-connect-to-the-proxy-server-error/
Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.


If an Answer is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.


0 Votes 0 ·

1 Answer

SusanDodds-3538 avatar image
0 Votes"
SusanDodds-3538 answered EricYin-MSFT edited

Internal DNS and port forwarding points to 2019.

What is causing the error? Is it just an error on the workstation side or does a server setting need changed?

· 1
5 |1600 characters needed characters left characters exceeded

Up to 10 attachments (including images) can be used with a maximum of 3.0 MiB each and 30.0 MiB total.

Hi,
Which version of Outlook are you using? Do all users have this issue? Are they using Win7 or Win10?
Please check the registry on those problematic users' PC:

 [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\WinHttp]

Is there a DWORD named DefaultSecureProtocols? If yes, try deleting it and restart the server, see if the issue solved.

Besides, is the cert installed on clients PC?

0 Votes 0 ·