Authentication Failure for IMAP and POP3 using Client Credential flow for OAuth2.0 | Java

Hi. I had the same problem, but I think I made some progress.

I read documentation few times, tried few times from the start with same error. I even have tried using client and object ids instead of email as username, in lack of better ideas.

So this is where I think I have made mistake previous times.

When you are at the part that you need to register service principal, you need to execute

New-ServicePrincipal -AppId <APPLICATION_ID> -ServiceId <OBJECT_ID> [-Organization <ORGANIZATION_ID>]

 here I put enterprise application object id as ServiceId argument. And that is ok.

But on 

Add-MailboxPermission -Identity "email address removed for privacy reasons" -User 
<SERVICE_PRINCIPAL_ID> -AccessRights FullAccess

I have put my registered application object id as User argument. I also tried setting object id of enterprise application, but it did not have success.

I also tried New-ServicePrincipal but with registered app object id as service id, but it gave me the same result.

When I executed 

Get-ServicePrincipal -Organization <ORGANIZATION_ID> | fl

I did not pay attention to ServiceId property, even with documentation specifying it and saying it will be different.

Now I cleared everything and started fresh.

I executed all the steps again, but on the step when I need to add mail permission, I list service principals, and then use `ServiceId` value from the output, as argument for user.

With that, I was able to authorise. 

Unfortunately, now I receive `C3 BAD User is authenticated but not connected.` when I try to list inbox. But it is step forward.

I am not sure if you made the same error as me, but maybe it will help you in some way.

I will post info, when I find fix for the new error if somebody comes across same issue.