Allow internet message headers that are not prefixed with X-
The /v1.0/me/sendMail endpoint allows the user to specify custom internet message headers, but requires that each header begin with “X-“ or “x-“. Please relax this requirement so that we can add other common headers like “List-Unsubscribe” and "List-Unsubscribe-Post".
Leon Mills commented
This is a pointless restriction that needs to be removed. I need to add this header as well for a client's small mailing list that he send to once a month. Why would this be restricted like this?