The email form automatically uses the email media server’s mail server configured in YourSite Explorer to submit emails to queues. To ensure Exchange uses the customer-entered email address in the form for responses and not the email mail server or queue email address, you must disable Outgoing SMTP Authentication in YourSite Explorer and configure authentication on the Exchange server.
For instructions on the specific Exchange configuration, see the following Mitel Knowledge Base article: https://mitel.custhelp.com/app/answers/answer_view/a_id/1001226/loc/en_US.