On the users_Registration_full.php page, the code for setting the recipient email is at line 149:
$RecipientEmail = "".((isset($_POST["UserEmail"]))?$_POST["UserEmail"]:"") ."";include("WA_Universal_Email/WAUE_users_Registration_1.php");
the form element that the user enters the email in is at line 385:
<input name="UserEmail" type="text" class="WAATKTextField" id="UserEmail" onBlur="if (document.getElementById('emailServerValidation')) document.getElementById('emailServerValidation').style.display='none'" value="<?php echo(ValidatedField("usersRegistrationfull","UserEmail")) ?>" size="32" />
you could verify that the recipient email is being set correctly by cvhnging line 149 to:
$RecipientEmail = "".((isset($_POST["UserEmail"]))?$_POST["UserEmail"]:"") ."";include("WA_Universal_Email/WAUE_users_Registration_1.php");
die("Email = ".$_POST["UserEmail"]);
this will cause the page to display a white screen showing the recipient email that is being used to send the email. If not, send a copy of your registration page so I can look into it.