Certain users view HTML NPrinting reports on mobile devices. (ex: iPhone)
Charts are embedded within the body of the message, displayed as images.
Upon forwarding the e-mail from their mobile device to other recipients, the format of the message changes, and all of the embedded chart images become e-mail attachments - corrupting the actual display of the message.
For example the message below was forwarded from an iPhone, and is what displays in Outlook.
Update - this problem is not unique to NPrinting. Many other HTML e-mail messages with embedded images will behave the same way, if viewing and/or forwarding using the default Apple Mail app within iOS. (ex: when using the "Outlook" app on the iPhone to forward the message to another recipient, the formatting stays intact)