Oh, congratulations. The recipient system is blocking you, possibly because your email meets THEIR spam criteria or because your IP (or ISP) is on a blacklist. You need to talk to their IT to find out exactly what their issue is.
Now, am I reading correctly that an email originating with your user goes through, but a reply does not? This could mean that they are reading the email or its format incorrectly. But either way, the problem is the way they are seeing these emails. I’ve had this problem before. Talk to the IT folks at the recipient company or ISP. The problem is on their end.