A noun would be implied from the context. Whatever was attached to the message e.g. a file, a document, a picture.
February 4, 2016
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It means that the sender of the email doesn't want you to say that he sent you the attachement to the email. e.g. it may be illegal for him to send it to you, or at the very least he shouldn't have sent it to you .
February 4, 2016
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