Cannot pass “In-Reply-To” parameter to Microsoft Graph sendMail
问题 I allow users to send emails with their Outlook account by using Microsoft Graph API, but it seems to be creating multiple threads on the other side. When using Mailgun API to send the user emails, I am able to pass the In-Reply-To message header that references the previous message Message-ID, and threads are clustered properly by clients on the other side (Outlook/Gmail etc) But when using Microsoft Graph API I try to pass the In-Reply-To and it is not accepted by the API graph_url = 'https