Hello,
the SMTP response "2.6.0 <80a1b0$b38@unknown domain> [InternalId=35171] Queued mail for delivery" are coming from the SMTP server to which the Appliance has delivered the message. This is not a error. It is only a Positive SMTP response from the destination smtp server that they have accepted the message and stored them within the message database with the Internal ID 35171.
If the Exchange admin are not able to see this message within the mailbox, they have to check the logs and search for the Internal Message ID 35171.
Further please also take a look on the complete mail_logs information from your Appliance. With the DCID information you should also be able to see to which IP address (SMTP Server) the Appliance has delivered the message.
If the Appliance has delivered the message to the wrong SMTP Server, you should take a look on the "SMTP Route" config of your Appliance to see if the route are correct for the internal Exchange server.
Best regards,
Mark.