iRule to modify SMTP mail content

Hi, We have an requirement, where in we need to inspect the incoming SMTP message, look for a particular string and based on that modify the mail content. I have written the below iRule but this is not working. The CLIENT_DATA event is being triggered. I refered to the SMTP proxy iRule provided in this link(https://devcentral.f5.com/wiki/iRules.SMTPProxy.ashx?NS=iRules). I am not sure if i am missing anything here.

 

when CLIENT_ACCEPTED {

 

TCP::respond "220\r\n" TCP::collect 2000 } when CLIENT_DATA { set cdata [TCP::payload] if { $cdata starts_with "DATA" } { if { [string match "Exchange2010R2" $cdata ]} {

 

string map {"Sent From:" "Sent From:\nExchange 2010 R2 Server\n"} TCP::payload

 

}

 

} TCP::release TCP::collect }

 

Thanks in Advance. Ajay