This is also begging for a format string, and the `%20`s make it hard to read.
```python
email_subject = "mailto:?subject={subject}&body={body}".format(
    subject="Take a course with {platform} online".format(platform=platform_name).replace(" ", "%20"),
    body="I just registered for {number} {title} through {platform} {url}".format(
        number=course.number,
        title=get_course_about_section(course, 'title'),
        platform=platform_name,
        url="http://{domain}{path}".format(
            domain=site_domain,
            path=reverse('about_course', args=[course.id]),
        )
    ).replace(" ", "%20")
)
```

This will also correctly replace spaces in the course number, course title, and platform name, where your code here will not (which will break the link).