office 365 outlook Graph api custom email message header - outlook

Question

In order to organize emails into single thread GMail requires custom headers to be set within incoming message. Is there any way to set custom headers when sending Office O365 Outlook message through the Graph API? The documentation states about custom headers option through the InternetMessageHeaders property, however it is required to start header name with 'x-' appendix, which makes it not usable. For example, this message is supposed to set some custom headers, however, due to the 'x-' prefix limitation, this can't be used for organizing GMail messages:
POST https://graph.microsoft.com/v1.0/me/sendMail
Content-type: application/json
{
"message": {
"subject": "9/9/2018: concert",
"body": {
"contentType": "HTML",
"content": "The group represents Nevada."
},
"toRecipients": [
{
"emailAddress": {
"address": "AlexW#contoso.OnMicrosoft.com"
}
}
],
"internetMessageHeaders":[
{
"name":"x-custom-header-group-name",
"value":"Nevada"
},
{
"name":"x-custom-header-group-id",
"value":"NV001"
}
]
}
}
https://learn.microsoft.com/en-us/graph/api/user-sendmail?view=graph-rest-1.0&tabs=http
Is there any way to specify custom headers: 'References', 'In-Reply-To' without the prefix 'x-'?
https://gsuiteupdates.googleblog.com/2019/03/threading-changes-in-gmail-conversation-view.html
When trying to set custom email headers without 'x-' prefix, the Graph API returns following error message:
{
"error": {
"code": "InvalidInternetMessageHeader",
"message": "The internet message header name 'References' should start with 'x-' or 'X-'.",
"innerError": {
"request-id": "441e21b7-d4ca-47d3-957a-a72bcc854a67",
"date": "2019-12-10T14:28:35"
}
}
}

Answer 1

One workaround might be to use the Extended properties to set those values eg the In-Reply-to https://learn.microsoft.com/en-us/office/client-developer/outlook/mapi/pidtaginreplytoid-canonical-property should look something like
"singleValueExtendedProperties": [
{
"id":"String 0x1042",
"value":"342342343234#domain.com"
}
]