What you show us is too few. You should check in Exchange the reason why messages have been refused.
It looks more a problem of DNS/navigation from Exchange server to qmailtoaster server, but I cannot say without knowing more.
I've got Exchange set to give me a higher level of logging for SMTP, but so far there is no mention. I'll check the DNS records for consistency.
What information would be helpful?
First of all, do you see in qmailtoaster server logs (/var/log/maillog is the usual place) any attempt of delivery from Exchange server?