Sending email with Microsoft graph API work account

Question

Does anyone know how to request permission to send emails using Graph API by an app that runs without a signed-in user?

I have an Azure WebApp with permission to send email using Microsoft Graph. In the Azure portal (Azure Active Directory -> App registrations -> MyApp - API permissions), my app has granted permission for Mail.Send( Type: Application : Description: Send mail as any user ).

In the next step, I’m inviting a user from my organization. In Azure Ad the user type is Guest. I receive an email on that account to accept the invitation. I can log in with that account through the Microsoft login page but the account is managed by my organization – it is not an account created by me.

Using that account with MS Graph explorer I’m able to send an email, but I want to do the same from my application without been logged in. The purpose is to use this account only for sending emails.

I was able to get the access token, use the API and get user basic info, but I get an exception when I'm trying to send an email:

Code: ResourceNotFound

Message: Resource could not be discovered.

// get token
var authContext =
    new AuthenticationContext("https://login.microsoftonline.com/" + tenantID);

var result = await authContext
    .AcquireTokenAsync("https://graph.microsoft.com", new ClientCredential(clientId, secret));

// create graph service
GraphServiceClient graphServiceClientApp =
    new GraphServiceClient("https://graph.microsoft.com/v1.0",
        new DelegateAuthenticationProvider(
            async(requestMessage) =>
            {
                requestMessage.Headers.Authorization =
                    new AuthenticationHeaderValue("bearer", result.AccessToken);
            }));

// create message obj
//.....

// send email
await graphServiceClientApp.Users["f5521fbc-481e-4e90-9166-33a64eb8f7e9"]
    .SendMail(message, false)
    .Request()
    .PostAsync();

The user ID like f5521fbc-481e-4e90-9166-33a64eb8f7e9 is taken from azure portal, in user details there is a Object ID field

Answer 1

When sending an email as a user, the user needs to have a mailbox associated with them.

Users without a license that includes Exchange Online, as well as external users (e.g. invited users) will generally not have a mailbox in the tenant, and thus would be unable to send emails.