Import this as a Postman collection to test Office365 message-getting. Don't forget to put in your own Bearer token, or another form of authentication.

Office365_GetInternetMessageHeaders.json

{
	"info": {
		"_postman_id": "084e1aa4-990f-47be-a872-8687ad49f7f8",
		"name": "Office365",
		"schema": "https://schema.getpostman.com/json/collection/v2.1.0/collection.json"
	},
	"item": [
		{
			"name": "Office365 StackOverflow thing",
			"request": {
				"auth": {
					"type": "bearer",
					"bearer": [
						{
							"key": "token",
							"value": "<BEARER TOKEN GOES HERE>",
							"type": "string"
						}
					]
				},
				"method": "GET",
				"header": [],
				"body": {
					"mode": "raw",
					"raw": ""
				},
				"url": {
					"raw": "https://outlook.office.com/api/beta/me/messages?$select=Id,Subject,InternetMessageHeaders",
					"protocol": "https",
					"host": [
						"outlook",
						"office",
						"com"
					],
					"path": [
						"api",
						"beta",
						"me",
						"messages"
					],
					"query": [
						{
							"key": "$select",
							"value": "Id,Subject,InternetMessageHeaders"
						}
					]
				},
				"description": "Get all email from Office365 and get the Internet Message Headers."
			},
			"response": []
		}
	]
}