Value from checkbox send by mail - PHP form

I have PHP form and there are checkboxes and I got something like that:

if(!empty($_POST['model_list'])) {
            foreach($_POST['model_list'] as $model) {
            }
        } else {
            throw new Exception('Choose the model.');
        }

In email that I got from this form is not shown what was choosed. Any idea what is wrong? When was something like that in the first post everything there was an information with value of checkbox.

Here is HTML used for every model

<input type="checkbox" value="A" name="model_list[]"><span>A</span>