Cannot send outgoing email via Office365: 5.7.57 SMTP; Client was not authenticated to send anonymous mail during MAIL FROM

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    Details

     

    When implementing Office365, outbound mail fails, and the error in the HEAT logs is:

    The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.57 SMTP; Client was not authenticated to send anonymous mail during MAIL FROM

     


    Resolution

    To resolve this issue, on the Office365 side, the mailbox should be changed from a Shared mailbox to a User mailbox.