How to block an ip address without DNS resolution in iptables

My server is under attacked by exim brute force with a fake ip:

2014-05-31 16:05:09 login authenticator failed for ([my_public_ip]) [178.41.179.50]: 535 Incorrect authentication data (set_id=grey)

I have some rules to block the origin 178.41.179.50 ip but no success. I try to block the incoming traffic with fake source with my public ip like this:

iptables -A INPUT -i eth0 -s my_public_ip -j DROP

But it seem take resolve my public ip to DNS instead.So it does not work.

Any help ?