Sending an email to several external addresses not working

We are trying to send out a promo flyer via email to serveral customers of ours. We keep getting the mail returned with an SMTP error and Relay access denied – reinject host refused rcpt to: – psmtp>

I belive their users mail handlers are rejecting this as spam mail perhaps.

Is there a way to send this? We could be sending this email out to as many as 40 people and increasing.

Thanks