Exchange 2010 and 2016 Coexistence - Mailbox hosted on EXCH2010 cant open Outlook

Question

Hello,

I have coexistence between EXCH2010 and 2016.
Client Access points to new EXCH2016 servers, i mean DNS names like outlook.domain.com, autodiscover.domain.com points to Exchange 2016.
I thought that EXCH2016 proxy to old EXCH2010 CAS if the mailbox is hosted in old 2010 Database, but it looks like it doesnt work :( .

IS there any tool, which can identify where the problem is?

Outlook 2016 said:
Unable to start microsoft outlook. The application window cannot be opened. The folder set cannot be opened. An attempt to log on to Microsoft Exchange failed

Thanks

Answer 1

Sounds like the ambiguous URL issue:
https://techcommunity.microsoft.com/t5/exchange-team-blog/ambiguous-urls-and-their-effect-on-exchange-2010-to-exchange/ba-p/593809

Answer 2

Hi @HanakJ ,

Mailbox hosted on EXCH2010 cant open Outlook. Owa for exchange 2010 users works fine.

Do you mean that those problematic users who cannot open Outlook can log on to their web mail normally?
Are those problematic users accessing their mailbox on devices which have already been added to their domain?

If they are trying internal access, in order to further confirm your issue, it is suggested for you to try the "Test E-mail AutoConfiguration" tool in Outlook to check if there anything wrong with the Autodiscover service. Please check the following steps:

1. While Outlook is running, hold down the CTRL key, right-click the Outlook icon in your system tray or notification area, and then select Test E-mail AutoConfiguration.
   
2. Enter the problematic email address, choose to use autodiscover, and click Test. Then check the information in the “Log” tab and see if there is any mistake or failure.
   

If the test result tells “Succeeded”, there might be something wrong with your connection between your Exchange server and Outlook client. While if the test result tells “Failed”, there might be something wrong with your Autodiscover configuration, you might need to get them check in detail. Please feel free to share the screenshots of your test result with us so that we could do further research. (Notice: Please hide your personal information in the screenshot for security.)

Any updates, please feel free to post back.

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