Mail Markup taps the new Gmail API to add a “Review Attachment” option to files in your Gmail account. You can draw with a pencil, add arrows or shapes and leave numbered annotations on the document. Text comments will then be pasted into an email draft with the marked-up image.
While the annotation options are fairly lightweight, they should be enough for most basic markup needs. If you and your coworkers email each other files for review on a daily basis, Mail Markup could save you a bit of time and trouble.
Apple is set to release its own Markup feature with the launch of OS X Yosemite this fall.