It's an issue on how the pictures are saved, and not much you can do about it, really. The rotation tag that can be read by a mobile device isn't properly translated when opening on a non-mobile device (or just plain stripped when attaching to e-mail). Depending on your device, the 'right' orientation might be taking a picture in portrait, might be landscape, might be with the phone facing left or facing right when in landscape... that might show up correctly on your device as the Gallery app reads the orientation tag correctly and automatically rotates the image, but this doesn't always happen when the image is sent/open elsewhere, so the recipient will see the original image orientation (again, depends on your device).
Using any editing app on the original camera image, however, should force the rotation to be overwritten.