If someone you're emailing through our app has Do not track (DNT) turned on in their web browser, you will not get a notification that they've read your email if they use a web-based email service. We believe that privacy should be a choice and if a recipient doesn't want to be tracked, we will give them tools that are available today to facilitate that.
However, having said that, most email apps (desktop clients) do not support any form of Do not track (DNT) but our systems are ready when that support is available. DNT is primarily a web-based technology today and only really works with web-based email services (Yahoo, Outlook.com, etc.)
A note about Gmail. Google recently introduced an image proxy for Gmail's web-based email. Google doesn't currently appear to pass the DNT header to us. When this is available, our DNT support will kick in.
Also in the future, we plan to add a facility on our website where people can opt out of all tracking by providing us their email address. We hope to have this shortly.