source: http://www.securityfocus.com/bid/1698/info

Like other firewalls, the Cisco PIX Firewall implements technology that reads the contents of packets passing through it for application-level filtering. In the case of SMTP, it can be configured so only certain smtp commands can be allowed through (for example, dropping extra functionality, such as HELP or commands that could be a security concern, like EXPN or VRFY). When recieving messages, it allows all text through between "data" and "<CR><LF><CR><LF>.<CR><LF>", as this is where the body of the message would normally go and there could be words in it that are smtp commands which shouldn't be filtered. Due to the nature of SMTP and flaws in exceptional condition handling of PIX, it is reportedly possible to evade the smtp command restrictions by tricking the firewall into thinking the body of the message is being sent when it isn't.

During communication with an smtp server, if the "data" command is sent before the more important information is sent, such as "rcpt to", the smtp server will return error 503, saying that rcpt was required. The firewall, however, thinks everything is alright and will let everything through until recieving "<CR><LF><CR><LF>.<CR><LF>". It is then possible for the attacker to do whatever he wishes on the email server. 

Here an example of what i could do exploiting this bug:
helo ciao
mail from: [email protected]
data ( From here pix disable fixup)
expn guest ( Now i could enumerate user
vrfy oracle and have access to all command)
help
whatever command i want
quit