There seems to be a wide spread documented conflict with Apple Mail and users on Telus ISP.
What transpires is that all "replied" and "forwarded" icons (the little arrows on the side of the message) do not appear in Apple Mail on Telus accounts.
There is a lengthy thread in the Apple Support Communities here:
Has anyone else experienced this? Is there a solution?
I've had the same problem for almost 2 years now. Have spent hours on the phone with Apple and they have been very patient--ran many tests, etc-- but confirm, it is not an Apple issue. I'm on my third operating system and the problem has followed me. Started in 2016, and have not been able to resolve it. Very frustrating as I have to cross-reference through In and Sent boxes to confirm if I replied or forwarded. A massive waste of my time. It does seem to be a Telus conflict with Apple mail program.