Email Bounces: Sender address rejected: Domain not found
The mail server mx.maxus.pl attempted to look up the domain name in the From: email address, [email protected], and was unable to find it.
This means your recipient's mail server is having trouble resolving domain names correctly, as I was able to look this up successfully.
The problem is with the recipient's mail system, not with your Google Apps setup. If the recipient fixes it before the expiration given in that warning message, then the mail will be delivered. Otherwise you will get a final bounce message after that time.