How do I see the user's name from message reply?

Hello,

I’ve got a reply from a user from sketchfab who I contacted via sketchfab.
Since the email does not show the user’s ID, I can not know who that was.

Where do I see the “sent messages” in Sketchfab web?

We don’t have such a list. But since you contacted that user in the first place, you should know who it is?