We have a dedicated, licensed user in Office 365 for our Acumatica email address.  I would definitely try to see if authorizing via a licensed user account resolves the issue (versus a shared mailbox).  That should be a fairly simple test to run.

Good luck!

@Marco Villasenor I found the diagnostic tool I was referring to.  It is https://testconnectivity.microsoft.com/tests/o365

This will allow you to verify if it is an Office 365 configuration or account issues versus an Acumatica issue.

We are getting this issue now but the emails are still being sent and no incoming processing has been configured… other than moving to modern auth was there ever a resolution on this?

I am running into problems setting up three email inboxes in our Acumatica tenant.  It appears as though I need to add an external application record for each email address?  I have also created an application for each email address in the Azure portal.  The instructions that I have been provided from my VAR seem outdated and incomplete (see attached). When I try and authenticate (ie: sign in), it appears as though the subsequent accounts that I have added appear to be trying to use the first account credentials some how.  Is there an updated documentation resource that better outlines the procedure of setting up Office 365 IMAP access with OAuth (modern authentication)?  Something isn’t right here and I feel like I have thoroughly reviewed everything that I can think of.  Admittedly I have less than one business days worth of experience setting up Acumatica.

You are correct that you need a separate external application record in Acumatica for each email however you only need one registered application in Azure for your organization.  Each External Application record in Acumatica can use the same client/tenant IDs from Azure as well as secret value,  Not sure that is the total problem but I know you don’t need multiple applications in Azure.  

As far as better documentation, I’ve used that same document you attached multiple times at multiple clients and was successful with setup.

Good luck!

Thanks for the clarification, that makes a lot more sense to me.  After updating my configuration, everything appears to be working.