This error means that a computer has attempted to deliver e-mail without a correct reverse DNS.
Due to the large quantity of spam messages, it has regrettably proven necessary to refuse invalidly configured e-mails mail servers.
Your system manager can rectify this by setting a correct reverse DNS for your e-mail server. If you do not know what this means, please send him or her the error message. This error message contains the address of the invalidly configured mail server.
A detailed description about setting your reverse DNS can be found here
Alternately, you can send the e-mail via your access provider’s SMTP-server. This usually is the smtp.provider.eu or mail.provider.eu.