By default the sender address for notifications looks like this: noreply@{fqdn}.
So in your case that would be: noreply@mydomaine.x.y.z.
However, I don’t believe that this is the end of story, because bad sender syntax indicates an incorrect mail address the system is trying to use.
You may want to check your FQDN setting as well as the email settings. You can adjust the sender there if needed.