Add a info box about needed dns names during let's encrypt mail creation

fixes #544

web/templates/admin/edit_mail.html

@@ -96,6 +96,10 @@
                                     <table style="display:<?php if ($v_ssl == 'no' ) { echo 'none';} else {echo 'block';}?> ;" id="ssltable">
                                         <tr>
                                             <td class="input-label vst-text">
+                                                <span class="alert alert-info alert-with-icon">
+                                                    <i class="fas fa-exclamation"></i>
+                                                    <?=__("Let's Encrypt for mail needs a dns record for mail.$v_domain and webmail.$v_domain!")?><br/>
+                                                </span>
                                                 <label><input type="checkbox" size="20" class="vst-checkbox" name="v_letsencrypt" <?php if($v_letsencrypt == 'yes' || $v_letencrypt == 'on') echo "checked=yes" ?> onclick="App.Actions.MAIL.toggle_letsencrypt(this)"> <?php print __('Lets Encrypt Support');?></label>
                                             </td>
                                         </tr>