(GUESS) If you don't have their app installed in Chrome and view within Chrome, you get emailed a link, which opens in a browser. Most likely it will be a rendered image (or something like that), though of course you could still attach that. (/GUESS)
If I receive a 'click on this to see your message', like many, I will probably email back whoever sent it, ask them to resend as a conventional email (that is, disable Dmail) or else I will simply delete it. Quite possibly I might consider writing an app which goes through my gmail via IMAP and automates this process (that is, scan inbox, detect dmail messages, auto-reply requesting conventional email, and move to dmail-spam).