I have Ubuntu server 12.04, Ispconfig 3 installed well and a static IP address. All is working well. But I guess my mail server is under atack If i enter tail -f /var/log/mail.log I get a lot of messages (2 per second) NOQUEUE: reject: RCPT from unknown[126.96.36.199]: 554 5.7.1 <firstname.lastname@example.org>: Relay access denied; from=<email@example.com> to=<firstname.lastname@example.org> proto=ESMTP helo=<50.1-200> domain.ro is one of my domains hosted on my server (my server have a static IP address) f6d6d1609 does not exist I've tried to block 188.8.131.52 with iptables but suddenly another ip appears making the same opperation. I think i have more than 100 ip`s blocked until now in the list. I guess somebody try to make spam with telnet using random mails from one of my domains. What can i do?