In the iOS Mail app, you can insert your own drawings. Hold down your finger in a message until you get the usual pop-up menu for pasting content or inserting a photo/video/attachment.
At the end of the list is Insert Drawing. The tools you get are the same as you get for annotating screenshots and PDFs—a marker, a highlighter, a pencil, eraser, a lasso for moving your annotations, and a color switcher (white, black, blue, green, yellow, or red). The + icon has some other options for inserting shapes and text in a drawing, even your signature.