yes, this is possible using the CF7 'wpcf7_mail_tag_replaced' filter,
add_filter('wpcf7_mail_tag_replaced', 'format_chackbox',10,4);
function format_chackbox($replaced, $submitted, $is_html, $mail_tag){
//you can check if this is the right field if need be.
if('my-checkbox-field' != $mail_tag->field_name()) return $replaced;
//$submitted contains the raw submitted data.
//$replaced is the formatted data, you can use either.
$a = explode(',', $replaced);
//check if you have multiple values and the email accepts html (set with the use html checkbox in the mail notification settings).
if(is_array($a) && $is_html){
$replaced = '<ul>'.PHP_EOL;
foreach($a as $v) $replaced .= '<li>'.trim($v).'</li>'.PHP_EOL;
$replaced .= '</ul>'.PHP_EOL;
}
return $replaced;
}
place this in your functions.php file