The account 'domain\PC696$' provided valid credentials, but it does not have submit permissions on SMTP Receive connector 'Default Frontend MX1'; failing authentication.
According to the error and the information in this link " Exchange 2013: Receive connector authentication error"(Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.), the error could because the receive connector "'Default Frontend MX1" doesn't allow authenticated relay of emails. Therefore, according to the solution in the above link, it is recommended that you try to check the "Exchange users" button and click "Save", then deliver the email messages again to check the result.
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