SharePoint is not a public site, and users need to be authenticated to get resources. Enter One Time Pin for SharePoint, an additional layer of security to harden any SharePoint site no matter On Premises or Office 365. This additional layer of security locks down a specific SharePoint site with two-factor authentication sending a One Time Pin to your phone via a mobile application.
This is the protection set by SharePoint for your information security. Per my knowledge, this currently can only be sent through the mobile app and not email.
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