An update is currently being deployed on SharePoint which will allows IT administrator to restrict external sharing to be validated only by the original email which sent the invitation.

To enable this, you must connect to your SharePoint tenant using PowerShell and set the RequireAcceptingAccountMatchInvitedAccount to TRUE (by default this is set to FALSE)

To check if your tenant is already updated, run the Get-SPOTenant and check if the parameter is present or not

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But stay tuned, more update is coming on SharePoint Online Smile