I did a fresh installation of 5.0.4, and it seems that the default templates for mail are broken.
If someone how has not the permission to write to a queue did it, the response mail failed with
“message has lines too long for transport”.
What did the response email look like? Is there something in the email that confirms it came from RT? That doesn’t sound like a message RT would send, so I suspect it may have come from the MTA that received the message.