Qmail attack

[Renato]

Hi all,

I was victim of an attack today. Somebody connected to my smtp server and 
sent multiple messages to same address. The headers look like:

From: "User" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>

Well, naturally somebody can connect to port 25 and send this mail with 
these headers. But the attacker used a script and sent the same message 
thousands of time !!! My queue grow to more than 10.000 messages in 
minutes !!

What can I do to avoid this type of attack ?

Thanks
Renato - Brazil.