First the checkboxes need to include "[]" at the end to create an array. for example:
html:
<input type="checkbox" name="Interests[]" value="Guided walks" id="Interests_3" />
Then to use the implode function, change:
php:
$MailBody = $MailBody . ((isset($_POST["Interests"]))?$_POST["Interests"]:"");
to:
php:
$MailBody = $MailBody . ((isset($_POST["Interests"]))?implode(", ", $_POST["Interests"]):"");
the first argument in the implode function is the separator. I used ", " to create a comma separated list.
