UPDATE: The InternetMessageHeaders
property was added to the beta endpoint of the Outlook API, so you can get this without using the extended property stuff. You do have to request the property explicitly via $select
though. Something like:
GET https://outlook.office.com/api/beta/me/mailfolders/inbox/messages?
$select=Subject,InternetMessageHeaders
For Graph: The property also exists on messages in the beta endpoint for Graph, so you can do:
GET https://graph.microsoft.com/beta/me/mailfolders/inbox/messages?
$select=subject,internetMessageHeaders
For non-beta endpoints: The API doesn’t directly provide access. However, you can access the PidTagTransportMessageHeaders MAPI property using the Extended Property API.
From the first link, we see that the property ID for PidTagTransportMessageHeaders
is 0x7D
, and the type is String
. So the $expand
parameter of your GET would look like:
$expand=SingleValueExtendedProperties($filter=PropertyId eq 'String 0x7D')
NOTE: This is only applicable for the Outlook endpoint (https://outlook.office.com). For Graph, see the answer from madsheep
Putting that together with a GET
for a specific message, your request might look like:
GET https://outlook.office.com/api/v2.0/me/messages/{message-id}?
$select=Subject,SingleValueExtendedProperties
&$expand=SingleValueExtendedProperties($filter=PropertyId eq 'String 0x7D')