Disable SMTP on Cisco firewall

Log on to the PIX device by establishing a telnet session or by using the console. 

Type enable, and then press ENTER. 
When you are prompted, type your password, and then press ENTER. 
Type "configure" terminal, and then press ENTER. 
Type "no fixup protocol smtp 25" , and then press ENTER. 
Type "write memory", and then press ENTER. 
Restart the PIX device, or reload the PIX configuration.

Please use above command carefully on your responsibility.