mailgun for loop doesn't render email correctly

Question

I have copied for loop example from mailgun documentation page and trying to make it work, but it doesn't recognize as a code

{% for item in items %}
      <span>{{ item }} </span>
{% endfor %}

the above code renders as

{% for item in items %} test_item {% endfor %}

in the email when it is sent or reviewed in the email review.

Do I need to enable something? {{name}} by itself works ok, loop doesn't work