Postfix has no bit-rate controls. If you want to reduce Postfix network bitrates, then use traffic shaping in your TCP/IP stack.
If you want to limit the mail volume to a Yahoo etc., use Postfix built-in concurrency limits. Configure dedicated SMTP clients in master.cf for Yahoo etc. and use reduced concurrency limit etc. values. /etc/postfix/master.cf: yahoo unix - - n - - smtp /etc/postfix/main.cf: yahoo_destination_concurrency_limit = 1 yahoo_destination_rate_delay = 1 yahoo_destination_recipient_limit = whatever Wietse