IIS FTP Brute Force Attack How To Prevent At Network Level

I am getting a few hundred IIS 6.0 FTP login attempts a second on my windows 2003 x64 server.

We have a Sonicwall TZ180, a full IPS and Firewall in front of the server but I cannot determine a way to block these attacks. I simply have port 25 open to all ip addresses, as I do not know a range of valid ips.

Is there any way to prevent these attacks at the firewall/hardware level? I suspect not, because the firewall doesn’t know if a login attempt is valid or not.

I have enabled IPS on the firewall but doesn’t appear to be stopping these attacks. Is there any way to automatically ban ips that hit port 25 X number of times in a second?