A la primera pregunta: Haz una lista en un array y cuando se registre un usuario añades su mail a la lista.
A lo otro: Tu manda el mail, si existe solo tiene que ir a la dirección que contiene el mail que tu mandas y que confirmara el registro. Si no responde en dos días, borras los datos y te olvidas.
Tambien puedes confirmar si el mail es verdadero verificando:Que la dirección por lo menos contenga 5 caracteres (Suma de u@d.c),
Que los caracteres antes y después de la arroba (@), sean letras, números y caracteres permitidos (".""_""-", etc).
Que incluya la arroba @
Que incluya por lo menos 1 punto (usuario@dominio"."com
También te dejo este código para confirmar (No he tenido tiempo de mirarmelo, asi que no se si funciona):
<?php
if (!$HTTP_POST_VARS){
echo "<html><body>
<form action=p2.php method=POST>
<input type=text name=mail>
<input type=submit name=boton value=Aceptar>
</form>
</html>";
}
else {
ValidateMail($mail);
// Step 1 -- Initialize the script
// The first step in any script, of course, is to initialize any variables you'll be using. In this case, declare any global variables you'll be using:
function ValidateMail($Email) {
global $HTTP_HOST;
$result = array();
// Step 2 -- Check the e-mail address format
// Next, you'll use our regular expression to determine if the e-mail address is properly formatted. If the e-mail address is not valid, return in error:
if (!eregi("^[_a-z0-9-]+(\.[_a-z0-9-]+)*@[a-z0-9-]+(\.[a-z0-9-]+)*(\.[a-z]{2,3})$", $Email)) {
$result[0]=false;
$result[1]="$Email is not properly formatted";
return $result;
}
// Step 3 -- Find the address of the mail server
// Now, split apart the e-mail address and use the domain name to search for a mail server you can use to further check the e-mail address. If no mail server is found, you'll just use the domain address as a mail server address:
// Note: In the event that the optional step 4 is not followed, the else portion of this step must return in error in order for the script to function properly.
list ( $Username, $Domain ) = split ("@",$Email);
if (getmxrr($Domain, $MXHost)) {
$ConnectAddress = $MXHost[0];
} else {
$ConnectAddress = $Domain;
}
// Step 4 -- Connect to mail server and check e-mail address (OPTIONAL)
// Finally, once you have the best guess at a mail server, it's time to open a connection and talk to the server. As I stated earlier, this step is optional. After every command you send, you'll need to read a kilobyte (1024 bytes) of data from the server. It should be more than enough to receive the complete response from the server for that command.
// Note that you'll store the output from the server in three separate variables: $To, $From and $Out. This is done so you can check the responses after you close the connection, to see if you actually have a real e-mail address or not.
// If the script cannot connect at all, or the e-mail address wasn't valid, set the $result array to the proper values:
$Connect = fsockopen ( $ConnectAddress, 25 );
if ($Connect) {
if (ereg("^220", $Out = fgets($Connect, 1024))) {
fputs ($Connect, "HELO $HTTP_HOST\r\n");
$Out = fgets ( $Connect, 1024 );
fputs ($Connect, "MAIL FROM: <{$Email}>\r\n");
$From = fgets ( $Connect, 1024 );
fputs ($Connect, "RCPT TO: <{$Email}>\r\n");
$To = fgets ($Connect, 1024);
fputs ($Connect, "QUIT\r\n");
fclose($Connect);
if (!ereg ("^250", $From) ||
!ereg ( "^250", $To )) {
$result[0]=false;
$result[1]="Server rejected address";
return $result;
}
} else {
$result[0] = false;
$result[1] = "No response from server";
return $result;
}
} else {
$result[0]=false;
$result[1]="Can not connect E-Mail server.";
return $result;
}
// Step 5 -- Return the results
// Finally, our last and easiest step is to return the results and finish:
$result[0]=true;
$result[1]="$Email appears to be valid.";
return $result;
} // end of function
}
?>
Un saludo!! 
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