Problems with sending email to freenet.de

Hello,

I have problems this sending emails to the freenet.de domain.
All mails goming back with :
550
inconsistent or no DNS PTR record for xxx.xxx.xxx.xxx (my ip address) (see RFC 1912 2.1) (in
reply to RCPT TO command)

I have multiple domains on 1 ip address, so i can only enter 1 RDNS per ip address.
Is there a way to solve this??

Henk