Unable to send mail

‘Your message did not reach some or all of the intended recipients.’
‘The following recipient(s) could not be reached:
‘loibl@law.co.at’
Unable to deliver the message due to a communication failure.
The MTS-ID of the original message is: c=US;a= ;p=KOFFELS;I=KOFFEL02-010608022213Z-1216.
Is this because of an invalid format of the e-mail? Why is it that the person ‘loibl@law.co.at’ can send mail to us, but we are unable to send mail back to him? We are able to send him mail from other locations, but not our office. We can send other mail fine, but not to that address.