I guess the first thing to do would be to work out what is different between the environments, If it works on one but not the other it would point to something environmental or that is at least where I would start.
Have you enabled debug logging to see if that shows anything?
By environments / environmental I mean that you mentioned it does work in a test environment but not the production environment so to me this would suggest there is something difference between the 2 and that would be worth investigating further.
I am also curious about why it only fails for one email or that is how I read the post, who is the mail provider for that address and do you have other emails with the same provider?
I found the error, when I create a new workflow with the same settings and nodes. It worked. But my old workflow still doesnt work. I dont know why, didnt setup logging since it would take some time…