How is it possible?

["Monty Ree" <chulmin2 () hotmail com>]

Hello, all.

I have operated smtp server and allow smtp function to my clients(over 
5,000) using one id/p auth.(smtp auth)

But, some user(I don't know him) sends lots of spam mails through this smtp 
server so I  have filtered that IP addr. then after some minutes he send 
spam mails again using other IP address which  is not same network range.
for example, 

He sends spams like this..

1.1.1.1 --> I filtered, then after 1-2 minutes later  
2.2.2.2 -->  I filtered, then after 1-2 minutes later  
3.3.3.3 -->  I filtered, then after 1-2 minutes later  
4.4.4.4 -->  I filtered, then after 1-2 minutes later  
5.5.5.5  ......

Surely, all IPs are one ISP network but network range is not same...

The spammer using proxy server? 

How is it possible? and how to solve against this attack?


Thanks in advance.

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