Spam Emails from So-Net

Recently, my email server got very heavy load of spams.
Just see a very short log file, lots from the IP address of SONET-TW (Sony Network Taiwan Limited).
I googled it and found it is a ISP company provide ADSL internet access service.
Because of so many load of server, I have to ban the IP address range as following: –

The spammers try to send spams through my email server. I turn off the open relay. They still try and try.
Even the server gave the 530 back. They doesn’t give up.

After ban these IP address, they can not access my email server any more.
It save my server load, and save my log files.
Note: I just removed my Imail email server and replaced by the hMailserver.
After ban the IP address above, the number of spam trying dropped.
I found a second IP address range from So-Net. –
I hope ban this IP range will lower the spam.