<div data-smart-links="true">

    <p><a href="mailto:">A link containing href[mailto:]</a></p>

</div>
{% extends "@link-page" %}
{% block main %}
  <p><a href="{{ modifier }}">A link containing href[{{ modifier }}]</a></p>
{% endblock %}
{
  "modifier": "mailto:"
}

Add the data-attribute data-smart-links="true" on the parent (normally your <main> element or the content article.