Run
sudo netstat -tulpn | grep mosquitto
on your raspberry that has mosquitto as server/broker.
Should show something like
tcp 0 0 0.0.0.0:1883 0.0.0.0:* LISTEN 3056/mosquitto
tcp6 0 0 :::1883 :::* LISTEN 3056/mosquitto
which means it is listening on port 1883 for ethernet and localhost.
If it just show 127.0.0.1 then it only listens to localhost which wold explain connection refuse for the remote host.
In case it does not list any process at all the process is not running. That cannot be the reason as the local client is able to publish a message.