Like many other organizations Graz University of Technology verifies if the sender (not the From:) address is replyable.
So you need to masquerade the sender addresses in some cases like PHP- or other webforms, which normaly use something like defaultname@hostname to send e-mails.
It depends on your MTA how to do this:
In sendmail.cf after lines S*11 and S*31
R$* $@ valid e-mail-address
or better read
In /etc/email-addresses you can define a sender address for local accounts:
# echo "myaccount: email@example.com" >> /etc/email-addresses
In Postfix you can fix it with masquerade_domains = …