Error - '-SmtpException: Transaction failed. The server response was: 5.2.252 SendAsDenied'

This error appears when Dimensions is configured to use SMTP for email and is trying to send the email as a user whose own SMTP account doesn’t have permission to Send As - hence SendAsDenied.
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In effect, though you're sending the emails you want to say that they're coming from someone else.
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So, for example, if emails are to be sent as coming from a different user than that of the actual Dimensions user then your IT needs to grant Send As permission on the Dimensions user's SMTP account. This is sometimes also known as delegate access permissions.