Forum Discussion
Full Mailbox Access & opening another users Maibox
This has been logged with MS, but wanted to come here and see if the community have found a way around this (using OWA is not an option)
Work in a 3rd Line support team for Exchange Online, occasionally we have end user outlook issues escalated to us.
To test/recreate/triage we often open the end users mailbox using outlook, in its own profile.
To do this, I would go into the EAC and assign my user account as Full Access to the users Mailbox. then create a new profile in outlook for the users mailbox only, opening it with my own Windows Credentials.
This action has worked as an Exchange Admin forever.
It stopped working for us early in 2025, likely due to basic auth deprecation.
We know that adding an additional mailbox to an existing profile will work, but that does not allow troubleshooting/testing/recreating end user problems or functions.
Now, when following the same steps, a Credential login prompt appears on screen for < 1sec and disappears
5 Replies
It should still work fine in classic Outlook, but if you have some form of SSO configured, it might be preventing you from getting to the "correct" login screen. The steps involved require you to go back one screen and change the UPN, but if you are getting auto-logged in, this will be a problem.
If you are using the new Outlook client, support for this method is still not rolled out.
I'd also advise you to add the Full Access permissions via PowerShell instead of the EAC, as with PowerShell you can specify the -Automapping:$false switch.
The Method isn't the issue.
Im aware of how to apply automapping.
Automapping isn;t in the picture here as we want to open the mailbox in its own profile using my own credentials.
Thanks
Hello, thanks for replying.
Its not the PowerShell or exchange permissions/access that is the problem. Using a very old version of Outlook (like 2016) still works.
Using classic outlook does not
we are using
Microsoft® Outlook® for Microsoft 365 MSO (Version 2408 Build 16.0.17928.20604) 64-bit
Stopped working for us in February.
SSO is not in the mix.
Proxy access to other people's mailboxes by granting full access.
This is exactly what we do.
I give my own user account Full Access to the mailbox.
