This article has been translated from the original Dutch version published by Femke Cornelissen in to English, with an explicit and upfront consent of Femke (find her on LinkedIn).
Except from the original Dutch content, also some Dutch screenshots have been replaced with an English version. And some extra notes are added for clarity.
Set an automatic out-of-office message with Power automate
You got a day off and you forgot to turn on your out-of-office (OOF) assistant in Outlook. Pretty recognizable, right?
With the out-of-office message, people who send you an email see that you are absent. But in addition, this is of course also reflected in Microsoft Teams. When you try to contact someone, there is a notification or you can recognize it by the presence icon of the profile picture in the chat.
It’s quite interesting that this process is quite easy to automate with Power Automate. When I’m OOF is set in my calendar, the next step (in this case, turn on absence) must be performed.
Issue: when opening Outlook and afterwards on a regular intervals afterwards, Outlook keeps prompting for a password multiple times (x5 or more), even when the password is correct.
The error/connection message is sent to the desktop foreground on top of other applications.
Even when the password is ok, the message is thrown again multiple times, when the Outlook client is checking for mail, at certain intervals…
[Solution Spoiler = configure the registry to enable ExcludeExplicitO365Endpoint, but there might be other options for your case…]
Product version
In this specific situation, the products below were involved. The issue might also apply to other versions
Office version= Microsoft 365
Outlook version Microsoft® Outlook® for Microsoft 365 MSO (Version 2109 Build 16.0.14430.20224) 64-bit
Exchange server version 15.1.2308.4008. (on premises)
Additional information
Type of mailbox
In this case, the issue was related to connecting to a functional/shared mailbox. Connection to the personal mailbox was working fine, at first sight.
Standalone vs Domain
In this particular case, the PC was not connected to the domain of the Exchange server.
But also important connection on Outlook from domain joined PC is ok, no reconnection message. [More on this at the end of the article, as the domain client had specific GPO policies configured, …]
Multiple mail accounts
Outlook connected to multiple mail accounts (so removing Outlook completely, was not really an option…)
No issue on phone
Connecting the same account on a smartphone, works fine.
Symptoms
Error message
No explicit error message but you get a window with
“Windows security
Microsoft Outlook
Connecting to <… mailbox …>
Remember my credentials”
Error screen
Troubleshooting
Account credentials
Tried to change password (password reset)
Tried to remove password in Microsoft credential manager
you might end up with a locked user account if you enter the wrong credentials by accident while outlook keeps popping up the password request. Better double check your password and better NOT enter it again, or change it in the password request. But you’ll get this request multiple times in a few seconds, that it can be quite annoying to get past it.
Mail account
Tried to reinstall the mail account.
Removed the mail account and reinstalled mail account.
Configuration panel – Mail profile
Create a new Outlook profile (do NOT remove the existing Outlook profile) and add ONLY the problematic account. Set it to ONLINE mode (disable caching mode)
You can manage this option via Control Panel > mail
Alternatively, when reinstalling the mail account in outlook, disable the option “Use cached Exchange Mode to download email to an Outlook data file”.
Check Outlook connection status
When Outlook is active, you’ll find an Outlook icon in the task bar…
To check the Outlook connection status you need to hold the CTRL button and then right click on the Outlook icon.
Then click “Connection Status…”
Check if you see the personal mailbox and shared mailbox connection.
Test Email AutoConfiguration…
When Outlook is active, you’ll find an Outlook icon in the task bar…
To check the Outlook connection status you need to hold the CTRL button and then right click on the Outlook icon.
Then click “Test Email AutoConfiguration…”
In the menu enter the mail address of the target mailbox, in this case it’s a share mailbox with a specific mail address.
Very likely you’ll see a bunch of autodiscover failures like:
Alternative – Network analysis with Fiddler
You can collect a network log with Fiddler or other network sniffer
The policy values that are defined the Autodiscover Process section can be either policy-based registry values or non–policy-based values. When they are deployed through GPO, or manual configuration of the policies key, the settings take precedence over the non-policy key. Non-Policy Key: HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\AutoDiscover Policy Key: HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\16.0\Outlook\AutoDiscover Each value is of type DWORD.
So to exclude Office365 checking point we add following key:
ExcludeExplicitO365Endpoint and set the value to 1.
This setting is registry for client only. Outlook will skip checking Office365 Endpoint for Autodiscover.
If you have already configured XML autodiscover it should not affect the existing setting as the information are stored in this XML file locally anyway so Outlook will know how to connect.
Outlook as priority always prefer local XML configuration. Then in case it cannot obtain certain data goes to another check point. So apart from first two steps Outlook 2016 implementation of Autodiscover (microsoft.com) there are checking points we can configure how Outlook should obtain certain information. We can disable them or force them.
You can give it a try if this won’t work as desired you can always revert the changes.
Always make a copy of your registry before you change anything in the registry.
There is no really any other way from the client perspective.
In our case we can see many redirections and autodiscover failures. Not sure why, looks like Outlook refers to some old data or old domain URLS or cannot obtain properly Autodiscover configuration file and it is trying different combinations to guess which link for Autodiscover is working. Once it calls for HTTPS Autodiscover of the correct link it gets timeouts… which might also indicate firewall issue or something.
Then it tries unencrypted HTTP and it succeeds. Now it redirects to Autodiscover configuration link. But it takes a few attempts to get there. That’s why you get multiple popups of the error message / or the password prompt.
Why the issue did not hit the domain joined mail clients?
The mail administrator had following options configured already:
For meeting requests in Microsoft Outlook, the program does not have a bcc (aka Blind copy) option to add participants to a meeting, without publishing all personal data (mail addresses) to the other participants.
Microsoft is aware of the issue, but hasn’t fixed the option yet.
Still you can request to have this option or request this function in Outlook, via Windows Feedback hub (hit the W10 Windows button, and type feedback) of via Microsoft Tech Community or Microsoft Q&A.
Visibility of participants to other participants
When you add participants to the “Required” or “Optional” section, they can see each others mail addresses. For smaller groups of people, that probably know each other, it’s not a big thing.
But for public events, this might be an issue. And certainly for large groups of participants, this is an overload of information.
And additionally, it might be considered as an inconvenience (or even a data breach) to publish data of other participants in a large group.
Limiting visibility to other participants
For matters of data protection it would be very handy to send the invite to the participants without exposing too much data.
Work around
As the bcc: option is missing, you can add people to the “Resources” option.
Steps
Create a new meeting request.
In the meeting options select, the “Required” or “Optional” button.
Then in the resources option, add the contacts or mail addresses of the participants.
Then add the required information to the invite, including online meeting options (Teams, …) and send the mail.
Alternative option : using iCAL file option via mail
Another option is
to create an meeting in your agenda,
add the required meeting details (including teams invite)
Identity Underground 
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