thanks for the tipp, but I finally get rid of the error yesterday
Obviously I had some misconfiguration with iptables on my vm-host.
Instead of:
iptables -t nat -A PREROUTING -p tcp --dport 25 -j DNAT --to 172.16.84.128:25
iptables -t nat -A PREROUTING -p tcp --dport 465 -j DNAT --to 172.16.84.128:465
I should have written:
iptables -t nat -A PREROUTING -p tcp -i eth0 --dport 25 -j DNAT --to 172.16.84.128:25
iptables -t nat -A PREROUTING -p tcp -i eth0 --dport 465 -j DNAT --to 172.16.84.128:465
(eth0 is the ethernet-port which connects to the internet).
Thanks to anybody who helped or at least tried to helped me.
Great regards,
fish