Exchange don't send email to second MX
It will never try the second server if it can connect to the server with the highest priority. It will only try the second server if it cannot connect to the first server.
The error appears to be a server resource error on the other side. Most times (not always) a different MX record will point to the same server via a different network path, so hitting the secondary IP address might result in the same error.
You should try and contact the administrator on the other end (Look at the SOA or WHOIS record) to see if they can help you with these errors.