iRule to modify SMTP mail content

Hi, Try this:

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

when CLIENT_DATA { 
    set cdata [TCP::payload] 
    log local0. "Old payload: $cdata"

    switch -glob $cdata {
        "DATA*" {
            if { $cdata ends_with "\r\n.\r\n" } {
                if { [string match "*Exchange2010R2*" $cdata ] } {
                    TCP::payload replace 0 [TCP::payload length] [string map {"Sent From:" "Sent From:\nExchange 2010 R2 Server\n"} [TCP::payload]]

                    log local0. "New payload: [TCP::payload]"
                }
                TCP::release 
            }
        }
        default {
            if { $cdata ends_with "\r\n" } {
                TCP::release
            }
        }
    }

    TCP::collect 
}

It works for me.