Creo que he encontrado una solución a tu problema.
en el archivo main.cf del postfix:
Code:
address_verify_poll_count (default: 3)
How many times to query the verify(8) service for the completion of an address verification request in progress.
The default poll count is 3.
Specify 1 to implement a crude form of greylisting, that is, always defer the first delivery request for a never seen before address.
Example:
address_verify_poll_count = 1
This feature is available in Postfix 2.1 and later.
address_verify_poll_delay (default: 3s)
The delay between queries for the completion of an address verification request in progress.
The default polling delay is 3 seconds.
Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).
This feature is available in Postfix 2.1 and later.
Lo he encontrado aquí:
http://www.postfix.org/postconf.5.html
No es para un dominio en especial, pero creo que aumentando el tiempo de 'delay' conseguirás que no se envien todos a la vez.
** Por si no te funciona, hazte un 'backup' o algo del archivo de main.cf, por si acaso, te lo digo porque no estoy muy seguro de que funcione.