Cannot pass "In-Reply-To" parameter to Microsoft Graph sendMail - microsoft-graph-api

Question

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://graph.microsoft.com/v1.0'
headers = {
'User-Agent': 'api/1.0',
'Authorization': f'Bearer {outlook_token}',
'Accept': 'application/json',
'Content-Type': 'application/json'
}
# Create recipient list in required format.
recipient_list = [{'emailAddress': {'name': name, 'address': address}} for name, address in recipients]
reply_to_list = [{'emailAddress': {'name': name, 'address': address}} for name, address in reply_to]
# Create email message in required format.
email_object = {
'message': {
'subject': subject,
'body': {
'contentType': content_type,
'content': body
},
'toRecipients': recipient_list,
'replyTo': reply_to_list,
'attachments': [{
'#odata.type': '#microsoft.graph.fileAttachment',
'contentBytes': b64_content.decode('utf-8'),
'contentType': mime_type,
'name': file.name
}],
'internetMessageHeaders': [
{
"name": "In-Reply-To",
"value": in_reply_to
},
]
},
'saveToSentItems': 'true'
}
# Do a POST to Graph's sendMail API and return the response.
request_url = f'{graph_url}/me/microsoft.graph.sendMail'
response = requests.post(url=request_url, headers=headers, json=email_object)
https://learn.microsoft.com/en-us/graph/api/user-sendmail?view=graph-rest-1.0
I get the following response:
{
"error": {
"code": "InvalidInternetMessageHeader",
"message": "The internet message header name 'in-Reply-To' should start with 'x-' or 'X-'.",
"innerError": {
"request-id": "7f82b9f5-c345-4744-9f21-7a0e9d75cb67",
"date": "2019-05-03T04:09:43"
}
}
}
Is there any way of having the emails sent in the same thread for recipients clients?

Answer 1

Even if this thread is old, maybe it will help someone:
With the latest graph version you can now also send e-mails with MIME content. With this API you can set the in-reply-to header and other header fields, which was not possible with the JSON upload.

Answer 2

You can't manipulate standard message headers this way. The internetMessageHeaders collection will only accept "custom headers". These are message headers that begin with x- i.e. x-some-custom-header).
In order to reply to a message, you need to use the /createReply endpoint:
POST https://graph.microsoft.com/v1.0/me/messages/{id-of-message}/createReply
This will generate a message with the appropriate headers. You can then update this message to add additional content/attachments before you send it:
PATCH https://graph.microsoft.com/v1.0/me/messages/{id-of-reply}
{
"body": {
"contentType": "HTML",
"content": body
}
}
POST https://graph.microsoft.com/v1.0/me/messages/{id-of-reply}/send