Set a custom header with Outlook/Office 365 REST

Question

I know it is possible to get the headers from an email with this Outlook REST, but is there a way that we can set custom headers as well? I can't seem to find any documentation.

Answer 1

Try setting the SingleValueExtendedProperties on the message object. You can try using the property id for PS_INTERNET_HEADERS - {00020386-0000-0000-C000-000000000046}.

Creating a new message with a custom header might look something like this:

POST https://outlook.office.com/api/beta/me/messages

Content-Type: application/json
{
    ...,
    "SingleValueExtendedProperties": [
        {
            "PropertyId":"String {00020386-0000-0000-C000-000000000046} Name x-my-custom-header",
            "Value":"Some Value"
        }
    ]
}