I was working on a project today to send an email with the Graph API. I discovered there was an issue with the API about a year ago, dropping the API. When I checked the email raw header, I could see the Reply-To was there in the proper format. Digging deeper, it turns out that Outlook sees the Reply-To but does not display it. However, if you click to reply to the message, the correct Reply_To shows up in the Send To field. I pass this along for those of you who may be attempting to use the Repy_To field. It may simply be that your mail reader is not displaying the field, but still storing it for reply emails.