App Store receipt email, forwarded as attachment, won't open New user--just switched from Windows to an iMac running Yosemite. In the Mail app I use 'Forward as Attachment' a lot, with no problems. 

However, I just forwarded an App Store email receipt as an attachment, and the recipient couldn't open the receipt. OS X identifies the attachment as a generic document (rather than an email) and can't find an app to open it. So far, this has only happened with the App Store receipt. 

Is this problem likely due to something odd about the format of App Store receipts? If that's the case, I'll simply export those receipts to PDF and attach those files. Or is there something fundamental about attachments in Mail that I don't understand?
App store receipts are multipart mails, first part is text/plain, second is text/html. So in theory forwarding them should work just fine, in practice I see the same behavior you are seeing.

So either forward them directly (not as attachment) or print a PDF first (which might be kind of hassle depending on how often you need to do it).