Hi @blam
Yes, you can send email on behalf of any user without having to manually add them to your tenant. Before doing so, make sure your application is registered as: Accounts in any organizational directory (Any Azure AD directory - Multitenant) and personal Microsoft accounts (e.g. Skype, Xbox).
Next use the auth code flow for interactive logins to get an access token, note that you need to change /{tenant id}
to /common
to allow personal accounts to log in.
https://login.microsoftonline.com/common/oauth2/v2.0/authorize?
client_id=client id
&response_type=code
&redirect_uri=redirect url
&response_mode=query
&scope=https://graph.microsoft.com/Mail.Send
&state=12345
Finally call the /sendMail endpoint to send email for the personal account.
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