Darragh,
that's not possible in the normal mail flow policy, as at the point the message exceeds the maximum size configured, the IronPort closes the connection with a soft bounce error, and the sending mailserver will generate the error message send to the sender. The only workaround to that would be to configure a greater mailsize in the mail flow policies, and later use a message or content filter with a message size condition. There you can configure a notification to be send to the recipient, and a bounce. If considering this please be aware that this workaround will use up performance from your appliance, as the large messages will be accepted first by the IronPort and then processed by the workqueue before bounced.
Regards,
Andreas