I had this discussion a couple of weeks ago with the Apple engineers.
Before iphone iOS update to 15.2:
When you reply to an HME, you see your address in the from field - BUT as this goes through Apple's system, it is hidden and the recepient does see only the HME.
AFTER Update IOS 15.2 and Montrrey 12.1 - these updates resolve this and you can see the HME when you reply!
i know I sent in feedback that the HME was incredibly valuable to see in the sent folder, so you can manage your activities. They probably already had this planned since it's in the last updates.
After the update I hit reply from an Amazon email I'm using HME currently and unique HME address will show.
My suggestion: get your updates and you can see and manage with confidence. Until that, it does work when you reply despite not seeing the HME. The only thing to be careful is if you have discussion threads with the merchant - be sure to keep those straight too as you usually would.