Adding text to Join Form ?

I have been getting few complains from new users that they are not receiving conformation emails from my site which does go mostly to their SPAM box. I am not sure who to control this.. But at the meantime, I'd like to add a notice txt on the JOIN FORM to tell the new users to check their SPAM box after they register to my site. Please see attached..  Could someone specify which file and code I need to modify ?

I wonder if everyone is having same issue and how you get around this..

Thank you !

just a quick thought - in templates/base/login_join_popup.html

<div class="sys-form-login-join" id="sys-form-login-join">
    <ul class="sys-flj-navigation">
        <li><a href="#tabs-login"><bx_text:_Login /></a></li>
        <li><a href="#tabs-join"><bx_text:_Join_now /></a></li>
    </ul>
    <div id="tabs-login">__top_menu____login_form__</div>
    <div id="tabs-join">__top_menu____join_form__</div>
    you can add text here - but need to format with css
</div>

Thank you so much Prof ! Great to see you back