As the recipient organization you can close off a mail so it no longer appears as an outstanding task.
Sometimes an organization leaves a project without closing out their mail. This can leave you with outstanding mail stuck in your tasks that you're unable to reply to, or close out yourself. The No Action Required mail status effectively allows you to clear off the outstanding task. This option is available to all recipient organizations. You'll need to have the Mark a mail as No-Action Required permission granted to your user role to perform this action.
Things to note: