Irule need to replace FQDN in SMTP traffic EHLO Message

when SERVER_CONNECTED {
    clientside { TCP::collect }
}
when CLIENT_DATA {
    set p [string tolower [TCP::payload]]
    if { $p starts_with "rcpt to:" } {
         put you condition here, for example, only apply if there is @example
        if { $p contains "@example>" } {
             use stream profile or string manipulation or regexp here...
        }
        TCP::release
    } else {
         something else, let it be released to server
        TCP::release
        TCP::collect
    }
}

when CLIENT_ACCEPTED {
    set chelo ""
    set cfrom ""
    set crcpt ""   
    TCP::respond "220\r\n"
    log local0. "client accepted"
    TCP::collect
}
when CLIENT_DATA {
    log local0. "payload: [TCP::payload]"
    set cdata [TCP::payload]
    if { [ string length $cdata ] <= 0 } {
        TCP::collect
        return
    }
    if { not ( $cdata contains "\r\n" ) } {
        log local0. "get <$cdata> so far"
        TCP::collect
        return
    }
    if { $cdata starts_with "HELO" || $cdata starts_with "EHLO" } {
        set chelo [TCP::payload]
        log local0. "get helo \[$cdata\]"
        TCP::respond "250 OK\r\n"
        TCP::payload replace 0 [string length $chelo] ""
        TCP::collect
        return
    }
    if { $cdata starts_with "MAIL FROM:" } {
        set cfrom [TCP::payload]
        log local0. "get from \[$cfrom\]"
        TCP::respond "250 OK\r\n"
        TCP::payload replace 0 [string length $cfrom] ""
        TCP::collect
        return
    }
    if { $cdata starts_with "RCPT TO:" } {
        set crcpt "$crcpt[TCP::payload]"
        log local0. "get rcpt \[$crcpt\]"
        TCP::respond "250 OK\r\n"
        TCP::payload replace 0 [string length [TCP::payload]] ""
        TCP::collect
        return
    }
    if { $cdata starts_with "DATA" } {
        log local0. "get data \[$cdata\]"
        TCP::payload replace 0 0 $chelo$cfrom$crcpt
    }
    TCP::release
    TCP::collect
}
when SERVER_CONNECTED {
    log "server connected"
    TCP::collect
}
when SERVER_DATA {
    log local0. "payload: [TCP::payload]"
    set sdata [TCP::payload]
    if { $sdata contains "\r\n354 " ||
            $sdata starts_with "354 " ||
            $sdata contains "354\r\n" } {
        log local0. "get resp \[$sdata\]"
        TCP::payload replace 0 [string length $sdata] "354 go ahead\r\n"
        TCP::release
    } else {
        TCP::collect
    }
}
when CLIENT_CLOSED {
    log local0. "client closed"
}