It looks like their mailer is supposed to forward it on inside a firewall or something, but something is broken. So their relay is reporting a relay error.

Not your problem, IOW.

Sometimes MTAs report "cute" error messages when things like this happen. They perhaps shouldn't as it can be confusing to figure out what's actually arong. It's an error message you shouldn't be seeing, but the intermediate relay has no way of knowing that.

Wade.