iRule to modify SMTP mail content

I haven't tested this but try something like this below. I would also recommend adding logging statements as you process through and get your iRule working correctly. This will be a timesaver as you troubleshoot and debug.

when CLIENT_ACCEPTED {
  TCP::respond "220\r\n" 
  TCP::collect 2000 
} 

when CLIENT_DATA { 
  set cdata [TCP::payload] 
  if { $cdata starts_with "DATA" } { 
    if { $cdata contains "Exchange2010R2" } {
      set new_payload [string map {"Sent From:" "Sent From:\nExchange 2010 R2 Server\n"} $cdata]
      TCP::payload replace 0 0 $new_payload
    }
  } 
  TCP::release 
  TCP::collect 
}

Seth