Send a message with Large Attachment

  Create a Draft message
  
  POST https://graph.microsoft.com/v1.0/users('gscales@.....com')/mailfolders/drafts/messages HTTP/1.1
  
  {
  "Subject": "Test Message 1234",
  "Body": {
    "ContentType": "HTML",
    "Content": "Rgds Glen"
  },
  "ToRecipients": [
    {
      "EmailAddress": {
        "Name": "glenscales@blahlah.com",
        "Address": "glenscales@blahlah.com"
      }
    }
  ]
}

Create the Attachment

POST https://graph.microsoft.com/v1.0/users('gscales@datarumble.com')/messages/AAMk..../attachments/createUploadSession

{
  "AttachmentItem": {
    "attachmentType": "file",
    "name": "rates.pdf",
    "size": 2377002
  }
}

Will return

{
  "@odata.context": "https://graph.microsoft.com/v1.0/$metadata#microsoft.graph.uploadSession",
  "uploadUrl": "https://outlook.office.com/api/gv1.0/users('150bb0...')/messages('AAM...')/AttachmentSessions('AAM...')?authtoken=eyJ....",
  "expirationDateTime": "2021-09-17T07:27:44.3599952Z",
  "nextExpectedRanges": [
    "0-"
  ]
}

Upload the attachment in chunks

PUT https://outlook.office.com/api/gv1.0/users('150b...4')/messages('AAM...')/AttachmentSessions('AAM....')?authtoken=ey...
Content-Range: bytes 0-399999/2377002

Then Send the draft message

POST https://graph.microsoft.com/v1.0/users('gscales@datarumble.com')/messages/AAM.../send