The following behavior is currently occurring more frequently with Outlook 2016 and Exchange Server 2016 CU7. Outlook asks for the login information after opening attachments or the password prompt appears.
I was able to reproduce this behavior in my environment with Outlook 2016 (Version 1708 Build 8431.2079) and Exchange 2016 CU7.
As soon as the attachment to an e-mail is opened in Outlook, the user is prompted to enter their user name and password. The attachment can be opened as usual. It doesn't seem to matter what type the attachment is. I was able to reproduce the problem with PDF and DOCX files with a test email:
Even after entering valid login data, the prompt appears again.
I first encountered the problem when I installed CU7 for Exchange Server 2016. However, I have received several messages that Exchange 2013 and Outlook 2013 are also affected by the problem.
In a test environment with Exchange 2016 CU7 and an older Outlook 2016 version, I was unable to reproduce the problem. Only after updating to build 8431.2079 did the error also occur in the test environment.
Apparently this is a bug in the interaction with the current Outlook version and the latest Exchange updates. I have not yet been able to identify a configuration problem on the Exchange server side. Therefore, only downgrading the Outlook version has helped me so far. I am using the Office 2016 Click-to-Run version and was able to switch to an older build with the following commands:
cd %programfiles%\Common Files\Microsoft Shared\ClickToRun\ officec2rclient.exe /update user updatetoversion=16.0.8326.2107
My Outlook/Office installation did not want to be downgraded to other builds. The corresponding build numbers can be found here:
Version and build numbers of update channel releases
Here are instructions for downgrading an Office Click-to-Run installation:
How to revert to an earlier version of Office 2013 or Office 2016 Click-to-Run
There is also a current thread in the Technet forum:
Outlook 2016 prompting for login credentials when opening any email attachments
With build 8326.2107 the problem no longer occurs for me. I assume that there will be an update soon...
Update 10/27/17:
Leider ist auch in der aktuellen Click-to-Run Version (8528.2139) das Problem immer noch vorhanden. Es gibt allerdings einen Workaround. Andi hat einen Hinweis im MS Forum gefunden, wonach das Problem mit einer Verbindung von Outlook zu outlook.office.com hervorgerufen wird (siehe Kommentare). Als Workaround wurde ein Eintrag in der Host-Datei des Clients vorgenommen um das Problem zu umschiffen. Ich habe es ebenfalls getestet und kann bestätigen, dass es hilft den Hostnamen für Outlook.office.com ins Leere laufen zu lassen. Ich würde allerdings vorschlagen, anstatt der lokalen HOST-Datei an jedem Client, den Eintrag am DNS Server zu manipulieren. Dazu kann einfach eine neue Zone mit dem Namen „Outlook.office.com“ angelegt werden, innerhalb der Zone wird dann ein A-Record ohne Namen mit dem Wert 127.0.0.1 angelegt:
This means that the host file does not have to be adapted on every client and it can be undone quickly. So far, Outlook has not asked me for my login information again. Thanks to Andi for the tip.
Update 10.01.2018
Currently the problem occurs again Outlook 2016 (Build 7730.2175) tries again the autodiscover query against Outlook.office365.com:
The workaround described above, which causes the Autodiscover query to fail, also works in this case.
Update 11.01.2018
Marc has pointed out that the autodiscover method has been changed in Outlook 2016 Click-to-Run. This completely passed me by. Here is the corresponding comment from Marc
Hello Franky
Hello Community,It's not a bug, it's a feature!
Wir haben nun nochmal ein wenig geforscht. Microsoft hat hier die Implementation von Autodiscover verändert und greift zunächst erstmal auf Office365 zu.
https://support.microsoft.com/en-us/help/3211279/outlook-2016-implementation-of-autodiscover
However, we also have computers in our company on which Autodiscover works properly. We have compared these once. On the computers where it does not work properly and it connects to O365, we have also stored 2 accounts under "File->Office account". One for the local Exchange account and one for the Microsoft Cloud service. Unfortunately, removing the cloud account was not successful. But maybe it is somehow related to this. Despite having the same version, our computers behave very differently here.
Schaut euch auch mal folgenden Link an:
https://www.gothamweb.com/portal/index.php/knowledgebase/8/Outlook-bypasses-AutoDiscover-and-connects-directly-to-Office-365-mailbox.htmlMit dem entsprechenden Registry Eintrag kann man Outlook davon abhalten den Autodiscover auf dem Office365 Server auszuführen:
HKEY_CURRENT_USER\SOFTWARE\Policies\Microsoft\office\16.0\outlook\autodiscover
DWORD: ExcludeExplicitO365Endpoint
Value = 1Das kann aber auch immer noch nicht die beste Lösung sein.
Kind regards
Marc Debus
Vielen Dank an Marc! Das geänderte Verhalten erklärt auch, warum „normale“ Outlook 2016 Installationen dieses Verhalten nicht an den Tag legen.