tool.medowar.de/mail-delivery-check.php

<?php
declare(strict_types=1);

/**
 * Works out where a sending mail server (MTA) would actually try to deliver mail
 * for a domain, following RFC 5321 §5.1 routing rules:
 *
 *   1. Look up the recipient domain's MX records and sort them by preference
 *      (lowest number = tried first). Equal preferences are chosen at random.
 *   2. Resolve each MX target to its A / AAAA addresses — those are the hosts a
 *      sender opens an SMTP connection to, in preference order.
 *   3. Null MX (RFC 7505): a single "0 ." record means the domain accepts no mail.
 *   4. Implicit MX (RFC 5321 §5.1): with no MX records, the domain's own A / AAAA
 *      records are used as an implicit MX at preference 0.
 *
 * The result is the ordered list of servers/IPs a sender would attempt, enriched
 * with ASN / country / company for each address.
 */
class DeliveryResolver
{
    /** @var string[] Human-readable notes / warnings about the routing. */
    public array $warnings = [];

    /** @var array<int,array{pref:int,host:string,implicit:bool,cname:?string,addresses:array<int,array{ip:string,version:string}>,error:?string}> */
    public array $targets = [];

    public bool $nullMx = false;   // RFC 7505 — domain explicitly refuses mail
    public bool $hasMx   = false;  // at least one usable MX record was found
    public ?string $error = null;

    public function resolve(string $domain): void
    {
        $mx = @dns_get_record($domain, DNS_MX) ?: [];

        // RFC 7505 Null MX: exactly one record, preference 0, target "." (root).
        if (count($mx) === 1
            && (int) ($mx[0]['pri'] ?? -1) === 0
            && rtrim((string) ($mx[0]['target'] ?? ''), '.') === '') {
            $this->nullMx = true;
            $this->warnings[] = 'This domain publishes a Null MX record (RFC 7505: "0 .") — '
                . 'it explicitly does not accept email. Senders should bounce immediately.';
            return;
        }

        if (!empty($mx)) {
            $this->hasMx = true;
            // Sort by preference ascending; senders try the lowest number first.
            usort($mx, static fn($a, $b) => ($a['pri'] ?? 0) <=> ($b['pri'] ?? 0));

            $prefs = array_map(static fn($r) => (int) ($r['pri'] ?? 0), $mx);
            if (count($prefs) !== count(array_unique($prefs))) {
                $this->warnings[] = 'Several MX records share the same preference. A sender picks '
                    . 'between equal-preference hosts at random, so the exact host order can vary per delivery.';
            }

            foreach ($mx as $r) {
                $host = rtrim((string) ($r['target'] ?? ''), '.');
                $this->targets[] = $this->buildTarget((int) ($r['pri'] ?? 0), $host, false);
            }
            return;
        }

        // No MX record → RFC 5321 implicit MX: try the domain's own A / AAAA.
        $implicit = $this->buildTarget(0, $domain, true);
        if (empty($implicit['addresses'])) {
            $this->error = 'No MX records and no A/AAAA records for the domain — '
                . 'there is nowhere to deliver mail. Senders will return a bounce.';
            return;
        }
        $this->warnings[] = 'No MX records found. Under RFC 5321 the domain\'s own address (A/AAAA) '
            . 'is used as an implicit MX at preference 0.';
        $this->targets[] = $implicit;
    }

    /**
     * Resolves one MX target to its addresses and flags common misconfigurations
     * (a CNAME where a hostname is required, or a target that does not resolve).
     */
    private function buildTarget(int $pref, string $host, bool $implicit): array
    {
        $target = [
            'pref'      => $pref,
            'host'      => $host,
            'implicit'  => $implicit,
            'cname'     => null,
            'addresses' => [],
            'error'     => null,
        ];

        if ($host === '') {
            $target['error'] = 'Empty MX target.';
            return $target;
        }

        // RFC 2181 §10.3 / RFC 5321 §5.1: an MX target must be a hostname with
        // address records, never a CNAME. Flag it, but still follow the chain.
        $cname = @dns_get_record($host, DNS_CNAME) ?: [];
        foreach ($cname as $c) {
            if (($c['host'] ?? '') === $host && !empty($c['target'])) {
                $target['cname'] = rtrim((string) $c['target'], '.');
                if (!$implicit) {
                    $this->warnings[] = sprintf(
                        'MX target "%s" is a CNAME pointing to "%s". RFC 2181 forbids this; some '
                        . 'senders reject such records. It should be an A/AAAA hostname.',
                        $host,
                        $target['cname']
                    );
                }
                break;
            }
        }

        foreach (@dns_get_record($host, DNS_A) ?: [] as $r) {
            if (!empty($r['ip'])) {
                $target['addresses'][] = ['ip' => $r['ip'], 'version' => 'IPv4'];
            }
        }
        foreach (@dns_get_record($host, DNS_AAAA) ?: [] as $r) {
            if (!empty($r['ipv6'])) {
                $target['addresses'][] = ['ip' => $r['ipv6'], 'version' => 'IPv6'];
            }
        }

        if (empty($target['addresses'])) {
            $target['error'] = $target['cname'] !== null
                ? 'Target is a CNAME and did not resolve to any address.'
                : 'MX host has no A/AAAA records — a sender cannot connect to it.';
        }

        return $target;
    }

    /** @return string[] Every unique IP across all targets, for batch enrichment. */
    public function allIps(): array
    {
        $ips = [];
        foreach ($this->targets as $t) {
            foreach ($t['addresses'] as $a) {
                $ips[] = $a['ip'];
            }
        }
        return array_values(array_unique($ips));
    }
}

/**
 * Enriches IPs with ASN / country / company via ip-api.com's free batch endpoint.
 * @param string[] $ips
 * @return array<string,array>
 */
function lookupIpInfo(array $ips): array
{
    $out = [];
    $ips = array_values(array_unique(array_filter($ips)));
    if (empty($ips)) {
        return $out;
    }
    $fields = 'query,status,message,country,countryCode,as,asname,isp,org,reverse';
    foreach (array_chunk($ips, 100) as $chunk) {
        $payload = array_map(static fn($ip) => ['query' => $ip, 'fields' => $fields], $chunk);
        $ch = curl_init('http://ip-api.com/batch');
        curl_setopt_array($ch, [
            CURLOPT_POST => true,
            CURLOPT_POSTFIELDS => json_encode($payload),
            CURLOPT_HTTPHEADER => ['Content-Type: application/json'],
            CURLOPT_RETURNTRANSFER => true,
            CURLOPT_TIMEOUT => 20,
        ]);
        $resp = curl_exec($ch);
        curl_close($ch);
        foreach ((json_decode((string) $resp, true) ?: []) as $item) {
            if (isset($item['query'])) {
                $out[$item['query']] = $item;
            }
        }
    }
    return $out;
}

$domain = trim((string) ($_GET['domain'] ?? ''));
$resolver = null;
$ipInfo = [];
$inputError = null;

if ($domain !== '') {
    // Accept a bare hostname; strip scheme/path if a URL was pasted, and an
    // email address if someone enters user@example.com.
    $domain = preg_replace('#^\w+://#', '', $domain);
    $domain = explode('/', $domain)[0];
    if (str_contains($domain, '@')) {
        $domain = substr($domain, strrpos($domain, '@') + 1);
    }
    $domain = strtolower(trim($domain));

    if (!preg_match('/^(?=.{1,253}$)([a-z0-9](-?[a-z0-9])*\.)+[a-z]{2,}$/', $domain)) {
        $inputError = 'Please enter a valid domain name (e.g. example.com) or an email address.';
    } else {
        $resolver = new DeliveryResolver();
        $resolver->resolve($domain);
        $ipInfo = lookupIpInfo($resolver->allIps());
    }
}
?>
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Mail Delivery Route Checker</title>
    <style>
        body {
            font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
            max-width: 1100px;
            margin: 40px auto;
            padding: 0 20px;
            background: #f5f5f5;
            color: #333;
        }
        h1 { color: #333; }
        .info { background: #e3f2fd; padding: 15px; border-radius: 5px; margin-bottom: 20px; }
        form.lookup { margin-bottom: 20px; display: flex; gap: 10px; flex-wrap: wrap; }
        input[type=text] {
            flex: 1; min-width: 240px; padding: 10px; font-size: 15px;
            border: 1px solid #ccc; border-radius: 5px;
        }
        .btn {
            display: inline-block; padding: 10px 20px; background: #2196F3; color: white;
            text-decoration: none; border-radius: 5px; border: none; cursor: pointer; font-size: 15px;
        }
        .btn:hover { background: #1976D2; }
        .error { background: #ffebee; color: #c62828; padding: 10px; border-radius: 5px; margin: 10px 0; }
        .warning {
            background: #fff8e1; color: #e65100; padding: 12px 15px; border-radius: 5px;
            margin: 10px 0; border-left: 5px solid #ff9800;
        }
        .verdict {
            display: flex; align-items: center; gap: 12px; padding: 16px 20px; border-radius: 6px;
            font-size: 17px; font-weight: 600; margin: 16px 0;
        }
        .verdict.ok   { background: #e8f5e9; color: #2e7d32; border-left: 6px solid #43a047; }
        .verdict.fail { background: #ffebee; color: #c62828; border-left: 6px solid #e53935; }
        h2 { color: #333; margin-top: 30px; }
        .target {
            background: white; border-radius: 6px; box-shadow: 0 1px 3px rgba(0,0,0,0.1);
            margin: 12px 0; padding: 14px 16px; border-left: 5px solid #2196F3;
        }
        .target.dead { border-left-color: #e53935; opacity: 0.85; }
        .target-head { display: flex; align-items: baseline; gap: 12px; flex-wrap: wrap; }
        .pref {
            display: inline-flex; align-items: center; justify-content: center; min-width: 30px; height: 30px;
            background: #2196F3; color: white; border-radius: 50%; font-weight: 700; font-size: 14px; padding: 0 6px;
        }
        .target.dead .pref { background: #e53935; }
        .mxhost { font-family: monospace; font-size: 16px; font-weight: 600; word-break: break-all; }
        .tag {
            display: inline-block; padding: 2px 8px; border-radius: 10px; font-size: 11px; font-weight: 700;
        }
        .tag.implicit { background: #ede7f6; color: #5e35b1; }
        .tag.cname { background: #fff3e0; color: #e65100; }
        .tag.first { background: #e8f5e9; color: #2e7d32; }
        .tag-err { color: #c62828; font-size: 13px; margin-top: 6px; }
        table { width: 100%; border-collapse: collapse; margin-top: 10px; }
        th, td { padding: 8px 12px; text-align: left; border-bottom: 1px solid #eee; font-size: 13px; }
        th { color: #555; background: #fafafa; font-weight: 600; }
        td.ip { font-family: monospace; }
        .ver { display: inline-block; padding: 1px 6px; border-radius: 3px; font-size: 11px; font-weight: 700; }
        .ver.IPv4 { background: #e3f2fd; color: #1565c0; }
        .ver.IPv6 { background: #f3e5f5; color: #6a1b9a; }
        .empty { text-align: center; color: #999; padding: 40px; }
        .muted { color: #888; font-size: 12px; }
        ol.flat { padding-left: 20px; }
        ol.flat li { margin: 4px 0; font-size: 14px; }
        ol.flat code { font-family: monospace; }
    </style>
</head>
<body>
    <h1>📮 Mail Delivery Route Checker</h1>

    <div class="info">
        Shows <strong>where a sending mail server would try to deliver</strong> email for a domain, following
        <strong>RFC 5321</strong> routing: MX records sorted by <strong>preference</strong> (lowest first), each MX
        resolved to its <strong>A / AAAA addresses</strong>, plus <strong>Null MX</strong> (RFC 7505) and
        <strong>implicit MX</strong> fallback. Every address is enriched with its ASN, country and company.
    </div>

    <form class="lookup" method="GET">
        <input type="text" name="domain" placeholder="example.com  (or user@example.com)"
               value="<?= htmlspecialchars($domain) ?>" autofocus>
        <button type="submit" class="btn">🔍 Trace delivery</button>
    </form>

    <?php if ($inputError): ?>
        <div class="error"><?= htmlspecialchars($inputError) ?></div>
    <?php endif; ?>

    <?php if ($resolver !== null): ?>

        <?php if ($resolver->nullMx): ?>
            <div class="verdict fail">🚫 <?= htmlspecialchars($domain) ?> does not accept mail (Null MX).</div>
        <?php elseif ($resolver->error !== null): ?>
            <div class="verdict fail">❌ Mail cannot be delivered to <?= htmlspecialchars($domain) ?>.</div>
        <?php else:
            $reachable = array_filter($resolver->targets, static fn($t) => !empty($t['addresses']));
        ?>
            <?php if (!empty($reachable)): ?>
                <div class="verdict ok">✅ Mail for <?= htmlspecialchars($domain) ?> would be delivered to
                    <?= count($reachable) ?> reachable <?= $resolver->hasMx ? 'MX host' : 'implicit MX host' ?><?= count($reachable) === 1 ? '' : 's' ?>.</div>
            <?php else: ?>
                <div class="verdict fail">❌ MX records exist but none resolve to a usable address.</div>
            <?php endif; ?>
        <?php endif; ?>

        <?php foreach ($resolver->warnings as $w): ?>
            <div class="warning">⚠️ <?= htmlspecialchars($w) ?></div>
        <?php endforeach; ?>

        <?php if ($resolver->error !== null): ?>
            <div class="error"><?= htmlspecialchars($resolver->error) ?></div>
        <?php endif; ?>

        <?php if (!empty($resolver->targets)): ?>
            <h2>Delivery targets, in the order a sender tries them</h2>
            <?php foreach ($resolver->targets as $i => $t): ?>
                <div class="target <?= empty($t['addresses']) ? 'dead' : '' ?>">
                    <div class="target-head">
                        <span class="pref" title="MX preference"><?= (int) $t['pref'] ?></span>
                        <span class="mxhost"><?= htmlspecialchars($t['host']) ?></span>
                        <?php if ($i === 0 && !empty($t['addresses'])): ?>
                            <span class="tag first">tried first</span>
                        <?php endif; ?>
                        <?php if ($t['implicit']): ?>
                            <span class="tag implicit">implicit MX (A/AAAA)</span>
                        <?php endif; ?>
                        <?php if ($t['cname'] !== null): ?>
                            <span class="tag cname">CNAME → <?= htmlspecialchars($t['cname']) ?></span>
                        <?php endif; ?>
                    </div>

                    <?php if ($t['error'] !== null): ?>
                        <div class="tag-err">⚠️ <?= htmlspecialchars($t['error']) ?></div>
                    <?php else: ?>
                        <table>
                            <thead>
                                <tr>
                                    <th style="width:60px;">#</th>
                                    <th>IP address</th>
                                    <th>Type</th>
                                    <th>PTR (reverse)</th>
                                    <th>ASN</th>
                                    <th>Company / ISP</th>
                                    <th>Country</th>
                                </tr>
                            </thead>
                            <tbody>
                                <?php foreach ($t['addresses'] as $j => $a): ?>
                                    <?php
                                        $info = $ipInfo[$a['ip']] ?? null;
                                        $ok = $info && ($info['status'] ?? '') === 'success';
                                        $asn = $ok ? ($info['as'] ?: '—') : '—';
                                        $company = $ok ? ($info['org'] ?: ($info['isp'] ?? '') ?: ($info['asname'] ?? '')) : '';
                                        $country = $ok ? trim(($info['country'] ?? '') . ' (' . ($info['countryCode'] ?? '') . ')', ' ()') : '';
                                        $ptr = $ok ? ($info['reverse'] ?? '') : '';
                                    ?>
                                    <tr>
                                        <td><?= $j + 1 ?></td>
                                        <td class="ip"><?= htmlspecialchars($a['ip']) ?></td>
                                        <td><span class="ver <?= $a['version'] ?>"><?= $a['version'] ?></span></td>
                                        <td class="ip"><?= htmlspecialchars($ptr ?: '—') ?></td>
                                        <td><?= htmlspecialchars($asn) ?></td>
                                        <td><?= htmlspecialchars($company ?: '—') ?></td>
                                        <td><?= htmlspecialchars($country ?: '—') ?></td>
                                    </tr>
                                <?php endforeach; ?>
                            </tbody>
                        </table>
                    <?php endif; ?>
                </div>
            <?php endforeach; ?>

            <?php
                // Flat "connection attempt" order across every reachable address.
                $flat = [];
                foreach ($resolver->targets as $t) {
                    foreach ($t['addresses'] as $a) {
                        $flat[] = ['pref' => $t['pref'], 'host' => $t['host'], 'ip' => $a['ip']];
                    }
                }
            ?>
            <?php if (!empty($flat)): ?>
                <h2>Connection attempt order</h2>
                <p class="muted">A sender opens an SMTP connection to these addresses in turn, moving on only when one is unreachable or defers.</p>
                <ol class="flat">
                    <?php foreach ($flat as $f): ?>
                        <li><code><?= htmlspecialchars($f['host']) ?></code> [pref <?= (int) $f['pref'] ?>] → <code><?= htmlspecialchars($f['ip']) ?></code></li>
                    <?php endforeach; ?>
                </ol>
                <p class="muted">
                    Note: equal-preference MX hosts, and the choice between IPv4/IPv6 per host, are ultimately up to
                    the sending MTA — so the exact order can differ between deliveries.
                </p>
            <?php endif; ?>

        <?php elseif (!$resolver->nullMx && $resolver->error === null): ?>
            <div class="empty">No delivery targets were found.</div>
        <?php endif; ?>

        <p class="muted" style="margin-top:20px;">IP intelligence via ip-api.com (free tier).</p>
    <?php endif; ?>
</body>
</html>