Note: This posting is not intended to be a tutorial on how to send email through REST on SharePoint but rather document the attempts to override the Reply-To within the REST service to prove that some headers cannot be added or overridden.
You’ll notice the lack of Reply-To. The From just makes the users display name appear next to the SharePoint no-reply email address. However the additional headers was tested to see if I could over-ride them. In my code I configured the object to be added to the REST call and tried overriding the Reply-To as well as Return-Path, including a Return-Pathg as an intended typo just to see if SharePoint would allow something (that wouldn’t be forced due to it being a non-standard header).
Then after sending it off, making sure the browser sent it as part of the REST payload:
Note the Reply-To and the Return-Path, the header additions do NOT override what SharePoint assigns them (the email@example.com address). However, other headers are definitely possible to add, as we see that the Return-Pathg intended typo header came through.