Graph API / Issue when external user send email - Error message : MailboxNotEnabledForRESTAPI

Question

Hello,

I would like to develop an application that allows users to send emails using their Office365 address.

I have two professional Microsoft accounts in different tenants. I created my application with the first account and invited my second account as an external user to the tenant. The application is registered as a multi-tenant app, and I’m using the endpoint https://login.microsoftonline.com/organizations/oauth2/v2.0/authorize to retrieve the authorization code.

When testing with my second account, everything works fine, and the application successfully sends emails. However, for most other external users I’ve invited, I encounter the error "MailboxNotEnabledForRESTAPI."

I’ve read on several forums that this could be a licensing issue because external users cannot use the license from their own tenant as they are considered part of the tenant where the app is registered.

Is it possible for external users to send emails through my application? And a second question, why are some external users, who don’t have licenses in my tenant, still able to send emails through my app?

Here are the permissions requested by the application: openid profile email offline_access Mail.Read Mail.ReadWrite Mail.Send User.Read