(connect to mail.xy.de[8x.xx.xx.xx]:25: No route to host)
Things to check;
- is a mail daemon running on the destination machine
- is it set to listen on the correct ip address and correct tcp port (do a "netstat -nlp" to check)
- if necessary restart the mail server on receiving machine - any errors in the log?
- is your networking config good (probably is, as you said you can send email to gmail)
- perhaps watch both mail logs (use "tail -f /var/log/maillog" or similar) and try a "postqueue -f" on the sending server to see if you can flush the queue. Look for any errors to show you why its not being delivered.