Where is my mistake or what is wrong?
What it means:
This message was created automatically by mail delivery software.
A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:
xxxxxxxxx@xxxx.xxx
SMTP error from remote mailer after MAIL FROM:<root@server1.xxxxxx.xxx>:
host mx0.xxx.xxx [xxx.xxx.xxx.xxx]: 550 5.1.8 {mx033-rz3} Cannot resolve your domain