I don't have memory of such behavior, but even then, this is in your receive pipeline, reading plain-text, publicly readable email meant for you and appending s

I don't have memory of such behavior, but even then, this is in your receive pipeline, reading plain-text, publicly readable email meant for you and appending something to its plain-text, publicly readable header meant for you, all in order for the server to present this information to you/your mail agent. I don't see why any of this would lead to any amount of concern, but feel free to present the header field you refer to.