How to integrate to external users outlook emails

Question

I'm building a SaaS that connects to customer's work/personal emails and listens to incoming email in the background (so they don't have to stay logged in) and runs an AI task on the email to give some data insights.

Some of the customers use Outlook as their email provider. How do I connect to a customer's Mailbox via Microsoft Graph? (NB: customers do not live in our tenant)

I've tried the Delegated User flow, but this doesn't work well for me as the key is short lived (1 hour) and I need to be able to access their emails even when they're not on the platform.

I've also tried the Application Only flow, but I keep getting a 401 status code when trying to access the email of someone outside my tenant/organisation (even if they're a guest).

Is there any plausible way to achieve this in Microsoft Graph? I know with Gmail it's pretty straight forward with their API.

Answer 1

Hi @Pro

This can be facilitated by application permissions, provided that your tenant is Tenant A and your client's tenant is Tenant B.

First, you need to register A multi-tenant application in Tenant A, as shown below:

Grant the application the necessary permissions, then request authorization from Tenant B's administrator using the following steps:

GET https://login.microsoftonline.com/{TenantB}/adminconsent
?client_id={cliend id of registered app in TenantA}
&state=12345
&redirect_uri={redirect_uri of registered app in TenantA}

Then get tenant B's access token:

POST https://login.microsoftonline.com/{TenantB}/oauth2/v2.0/token
client_id={cliend id of registered app in TenantA}
&scope=https%3A%2F%2Fgraph.microsoft.com%2F.default 
&client_secret={client_secret}
&grant_type=client_credentials

Hope this helps.

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