/var/www/mail/config-v1.1.xml.php

<?php
$emailaddress = $_GET["emailaddress"];
$username = "%EMAILADDRESS%";
$password = "";

switch ($emailaddress) {
    case "foo@example.com":
        $username = "foo";
        $password = "pass1234";
        break;
}

header('Content-type: text/xml');
?>



    example.com
    example.net

    Foo Inc.
    mr


        imap.example.com
        143
        STARTTLS
        password-cleartext
        <?php echo($username); ?>
        <?php echo($password); ?>



        smtp.example.com
        25
        STARTTLS
        password-cleartext
        <?php echo($username); ?>
        <?php echo($password); ?>

Links
https://www.heise.de/ct/ausgabe/2013-8-Selbstkonfiguration-von-E-Mail-Clients-2324925.html
https://developer.mozilla.org/de/docs/Mozilla/Thunderbird/Autokonfiguration