Error [404] The mailbox is either inactive, soft-deleted, or is hosted on-premise while using Make/integromat

Question

Hello pros,

I'm working on a Make automation for a client, and we want to send messages via their company's Outlook mailbox. They have a Microsoft Partner provider who is supposed to configure Outlook and set up an address nepasrepondre@entreprise.fr so that I can send a message, but they can't manage it. So I did tests on my side with a sandbox, but I'm encountering the same error.

Here's what I did:

Creation of the automation:

• I created an automation on Make to send an email using my client's Outlook mailbox.

Configuration on Microsoft Entra/Azure:

• I created an application by specifying the redirect URI and assigning the appropriate API permissions (Mail.Send, offline_access, User.Read).
• I assigned my account to this application and obtained the Client ID, Secret ID, and Tenant ID.
• I managed to connect to the Outlook module on Make, and everything works correctly on my side.

Setting up the shared mailbox:

• I created a shared address nepasrepondre@entreprise.fr.
• I invited an external account contact@soustraitant.com to create an account (different domain).
• Once the external account was created, I attached it to the shared address nepasrepondre@entreprise.fr so that I can send emails from this address.
• I also assigned an Outlook license to these two accounts.

I also tried different things:

• Changing the status of the address contact@soustraitant.com from "Guest" to "Member".
• Switching the account from external to internal.
• Etc.

The problem:

When I attempt to send an email via Make using the shared mailbox, I receive the following error:

Error [404] The mailbox is either inactive, soft-deleted, or is hosted on-premise

If I connect directly with an account firstname.lastname@entreprise.fr (who is an admin) via this Microsoft application, it works. I can send emails with the address nepasrepondre@entreprise.fr.

On the mailbox nepasrepondre@entreprise.fr, I have also given permissions to the account contact@soustraitant.com (read permissions, manage, send as, etc.).

My questions:

• Has anyone encountered this type of problem before?
• Are there specific configurations or steps that I may have missed to allow sending emails via the shared mailbox from Make?

Thank you in advance for your help! 🙏