I'm running an Exchange 2003 server on a Telstra Business internet connection (in Australia).

I'm routing emails through the SMTP connecter, using the host smtp.telstrabusiness.com

When users send single emails to a single recipient, the emails are sent correctly.

However, when users send emails to a distribution list, they bounce back with:

The message could not be delivered because the recipient's destination email system is unknown or invalid. Please check the address and try again, or contact your system administrator to verify connectivity to the email system of the recipient.

One of the people in the list was able to recieve a single email from us just a few minutes afterwards, but did not recieve any of the email that should have come from the distribution list.

Is there a way I can throttle the number of messages that Exchange will send at a time, so as not to appear as spam to Telstra?