diff --git a/composer.json b/composer.json
index 3c5ffa8..35b42b4 100644
--- a/composer.json
+++ b/composer.json
@@ -24,6 +24,7 @@
     "require": {
         "php": "^8.0",
         "ext-curl": "*",
+        "ext-intl": "*",
         "ext-json": "*",
         "ext-openssl": "*",
         "composer/ca-bundle": "^1.3"
diff --git a/src/API/Endpoints/BaseEndpoint.php b/src/API/Endpoints/BaseEndpoint.php
index fb1b534..5312de5 100644
--- a/src/API/Endpoints/BaseEndpoint.php
+++ b/src/API/Endpoints/BaseEndpoint.php
@@ -10,6 +10,7 @@
 use Vatly\API\Resources\Links\LinksResourceFactory;
 use Vatly\API\Resources\Links\PaginationLinks;
 use Vatly\API\Resources\ResourceFactory;
+use Vatly\API\Support\IdnEmail;
 use Vatly\API\VatlyApiClient;
 
 abstract class BaseEndpoint
@@ -228,7 +229,7 @@ protected function parseRequestBody(array $body): ?string
             return null;
         }
 
-        return @json_encode($body);
+        return @json_encode(IdnEmail::normalizePayload($body));
     }
 
     /**
diff --git a/src/API/Endpoints/OrderEndpoint.php b/src/API/Endpoints/OrderEndpoint.php
index 58d108d..e5ee7e0 100644
--- a/src/API/Endpoints/OrderEndpoint.php
+++ b/src/API/Endpoints/OrderEndpoint.php
@@ -58,12 +58,11 @@ public function requestAddressUpdateLink(string $id, array $data = []): Link
 
         $resource = "{$this->getResourcePath()}/" . urlencode($id) . "/request-address-update-link";
 
-        $body = null;
-        if (count($data) > 0) {
-            $body = json_encode($data);
-        }
-
-        $result = $this->client->performHttpCall(self::REST_CREATE, $resource, $body);
+        $result = $this->client->performHttpCall(
+            self::REST_CREATE,
+            $resource,
+            $this->parseRequestBody($data),
+        );
 
         return new Link($result->href, $result->type);
     }
diff --git a/src/API/Endpoints/SubscriptionEndpoint.php b/src/API/Endpoints/SubscriptionEndpoint.php
index 426ed23..0345215 100644
--- a/src/API/Endpoints/SubscriptionEndpoint.php
+++ b/src/API/Endpoints/SubscriptionEndpoint.php
@@ -91,12 +91,11 @@ public function updateBilling(string $subscriptionId, array $data = []): Link
 
         $resource = "{$this->getResourcePath()}/" . urlencode($subscriptionId) . "/update-billing";
 
-        $body = null;
-        if (count($data) > 0) {
-            $body = json_encode($data);
-        }
-
-        $result = $this->client->performHttpCall(self::REST_UPDATE, $resource, $body);
+        $result = $this->client->performHttpCall(
+            self::REST_UPDATE,
+            $resource,
+            $this->parseRequestBody($data),
+        );
 
         return new Link($result->href, $result->type);
     }
diff --git a/src/API/Support/IdnEmail.php b/src/API/Support/IdnEmail.php
new file mode 100644
index 0000000..636cbb3
--- /dev/null
+++ b/src/API/Support/IdnEmail.php
@@ -0,0 +1,74 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Vatly\API\Support;
+
+class IdnEmail
+{
+    /**
+     * Convert the domain part of an email to its Punycode (ASCII) form.
+     *
+     * The local-part is returned unchanged: there is no equivalent ASCII
+     * encoding for non-ASCII local-parts, and the Vatly API does not
+     * support EAI / SMTPUTF8 (see openapi.yaml).
+     *
+     * Fail-open: if the input has no '@' or idn_to_ascii fails, return
+     * the original value so the caller sees the server's validation error
+     * rather than a silently mangled address.
+     */
+    public static function toAscii(string $email): string
+    {
+        $at = strrpos($email, '@');
+        if ($at === false) {
+            return $email;
+        }
+
+        $local = substr($email, 0, $at);
+        $domain = substr($email, $at + 1);
+
+        if ($domain === '') {
+            return $email;
+        }
+
+        $asciiDomain = idn_to_ascii($domain, IDNA_NONTRANSITIONAL_TO_ASCII, INTL_IDNA_VARIANT_UTS46);
+        if ($asciiDomain === false) {
+            return $email;
+        }
+
+        return $local . '@' . $asciiDomain;
+    }
+
+    /**
+     * Walk a request payload and normalize any string value at a key named
+     * 'email' to its Punycode (ASCII) form. Recurses into nested arrays.
+     *
+     * The `metadata` key is treated as opaque: the OpenAPI spec defines it
+     * as arbitrary application-defined key/value data, so the SDK must not
+     * rewrite values nested inside it (e.g. a caller storing their own
+     * `metadata.email` key should get it through to the API untouched).
+     *
+     * @param array<int|string, mixed> $payload
+     * @return array<int|string, mixed>
+     */
+    public static function normalizePayload(array $payload): array
+    {
+        foreach ($payload as $key => $value) {
+            if ($key === 'metadata') {
+                continue;
+            }
+
+            if (is_array($value)) {
+                $payload[$key] = self::normalizePayload($value);
+
+                continue;
+            }
+
+            if ($key === 'email' && is_string($value)) {
+                $payload[$key] = self::toAscii($value);
+            }
+        }
+
+        return $payload;
+    }
+}
diff --git a/tests/Endpoints/CustomersEndpointTest.php b/tests/Endpoints/CustomersEndpointTest.php
index 41fe730..99e8b39 100644
--- a/tests/Endpoints/CustomersEndpointTest.php
+++ b/tests/Endpoints/CustomersEndpointTest.php
@@ -95,6 +95,37 @@ public function it_creates_customer_with_minimal_data(): void
         $this->assertNull($customer->metadata);
     }
 
+    /** @test */
+    public function it_punycodes_the_email_domain_before_sending(): void
+    {
+        $responseBodyArray = [
+            'id' => 'customer_78b146a7de7d417e9d68d7e6ef193d18',
+            'resource' => 'customer',
+            'email' => 'user@xn--mnchen-3ya.de',
+            'createdAt' => '2020-01-01T00:00:00+00:00',
+            'testmode' => true,
+            'links' => [
+                'self' => [
+                    'href' => self::API_ENDPOINT_URL . '/customers/customer_78b146a7de7d417e9d68d7e6ef193d18',
+                    'type' => 'application/hal+json',
+                ],
+            ],
+        ];
+
+        $this->httpClient->setSendReturnObjectFromArray($responseBodyArray);
+
+        $this->client->customers->create([
+            'email' => 'user@münchen.de',
+        ]);
+
+        $this->assertWasSentOnly(
+            VatlyApiClient::HTTP_POST,
+            self::API_ENDPOINT_URL . '/customers',
+            [],
+            '{"email":"user@xn--mnchen-3ya.de"}'
+        );
+    }
+
     /** @test */
     public function it_can_get_a_customer(): void
     {
diff --git a/tests/Endpoints/OrderEndpointTest.php b/tests/Endpoints/OrderEndpointTest.php
index 12edc9b..5740fe1 100644
--- a/tests/Endpoints/OrderEndpointTest.php
+++ b/tests/Endpoints/OrderEndpointTest.php
@@ -287,4 +287,27 @@ public function can_request_address_update_link(): void
         );
         $this->assertEquals(self::API_ENDPOINT_URL."/customer/invoices/$orderId?editable=true&signature=1234567890abcdef", $response->href);
     }
+
+    /** @test */
+    public function request_address_update_link_punycodes_email_domains_in_billing_address(): void
+    {
+        $orderId = 'order_dummy_id';
+        $this->httpClient->setSendReturnObjectFromArray([
+            'href' => self::API_ENDPOINT_URL."/customer/invoices/$orderId?editable=true&signature=1234567890abcdef",
+            'type' => 'text/html',
+        ]);
+
+        $this->client->orders->requestAddressUpdateLink($orderId, [
+            'billingAddress' => [
+                'email' => 'billing@müller.de',
+            ],
+        ]);
+
+        $this->assertWasSentOnly(
+            VatlyApiClient::HTTP_POST,
+            self::API_ENDPOINT_URL.'/orders/'.$orderId.'/request-address-update-link',
+            [],
+            '{"billingAddress":{"email":"billing@xn--mller-kva.de"}}'
+        );
+    }
 }
diff --git a/tests/Endpoints/SubscriptionEndpointTest.php b/tests/Endpoints/SubscriptionEndpointTest.php
index 9f4f01e..cea55e0 100644
--- a/tests/Endpoints/SubscriptionEndpointTest.php
+++ b/tests/Endpoints/SubscriptionEndpointTest.php
@@ -246,6 +246,29 @@ public function can_update_billing_details()
         $this->assertEquals(self::WEBSITE_ENDPOINT_URL.'/checkout/checkout_dummy_id/update', $response->href);
     }
 
+    /** @test */
+    public function update_billing_punycodes_email_domains_in_billing_address(): void
+    {
+        $this->httpClient->setSendReturnObjectFromArray([
+            'href' => self::WEBSITE_ENDPOINT_URL.'/checkout/checkout_dummy_id/update',
+            'type' => 'text/html',
+        ]);
+
+        $this->client->subscriptions->updateBilling('subscription_123', [
+            'billingAddress' => [
+                'email' => 'billing@müller.de',
+                'city' => 'München',
+            ],
+        ]);
+
+        $this->assertWasSentOnly(
+            VatlyApiClient::HTTP_PATCH,
+            self::API_ENDPOINT_URL.'/subscriptions/subscription_123/update-billing',
+            [],
+            '{"billingAddress":{"email":"billing@xn--mller-kva.de","city":"München"}}'
+        );
+    }
+
     /** @test */
     public function can_update_subscription_quantity()
     {
diff --git a/tests/Support/IdnEmailTest.php b/tests/Support/IdnEmailTest.php
new file mode 100644
index 0000000..7de505a
--- /dev/null
+++ b/tests/Support/IdnEmailTest.php
@@ -0,0 +1,117 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Vatly\Tests\Support;
+
+use PHPUnit\Framework\TestCase;
+use Vatly\API\Support\IdnEmail;
+
+class IdnEmailTest extends TestCase
+{
+    /** @test */
+    public function it_passes_ascii_addresses_through_unchanged(): void
+    {
+        $this->assertSame('user@example.com', IdnEmail::toAscii('user@example.com'));
+    }
+
+    /** @test */
+    public function it_converts_idn_domain_to_punycode(): void
+    {
+        $this->assertSame('user@xn--mnchen-3ya.de', IdnEmail::toAscii('user@münchen.de'));
+        $this->assertSame('billing@xn--mller-kva.de', IdnEmail::toAscii('billing@müller.de'));
+    }
+
+    /** @test */
+    public function it_returns_input_unchanged_when_at_sign_is_missing(): void
+    {
+        $this->assertSame('not-an-email', IdnEmail::toAscii('not-an-email'));
+        $this->assertSame('', IdnEmail::toAscii(''));
+    }
+
+    /** @test */
+    public function it_returns_input_unchanged_when_conversion_fails(): void
+    {
+        // Empty domain — idn_to_ascii returns false on this. Fail-open: return original.
+        $this->assertSame('user@', IdnEmail::toAscii('user@'));
+    }
+
+    /** @test */
+    public function it_splits_on_the_last_at_sign(): void
+    {
+        // Quoted local-parts may contain '@'; we should split on the rightmost one.
+        $this->assertSame('"foo@bar"@xn--mnchen-3ya.de', IdnEmail::toAscii('"foo@bar"@münchen.de'));
+    }
+
+    /** @test */
+    public function it_leaves_unicode_local_part_untouched(): void
+    {
+        // No standardized ASCII encoding for non-ASCII local-parts. Server rejects
+        // these per the API spec; the SDK should not silently transform them.
+        $this->assertSame('jösé@xn--mnchen-3ya.de', IdnEmail::toAscii('jösé@münchen.de'));
+    }
+
+    /** @test */
+    public function normalize_payload_walks_nested_structures(): void
+    {
+        $input = [
+            'email' => 'user@münchen.de',
+            'billingAddress' => [
+                'email' => 'billing@müller.de',
+                'city' => 'München',
+            ],
+            'metadata' => [
+                'order_id' => '123',
+            ],
+        ];
+
+        $expected = [
+            'email' => 'user@xn--mnchen-3ya.de',
+            'billingAddress' => [
+                'email' => 'billing@xn--mller-kva.de',
+                'city' => 'München',
+            ],
+            'metadata' => [
+                'order_id' => '123',
+            ],
+        ];
+
+        $this->assertSame($expected, IdnEmail::normalizePayload($input));
+    }
+
+    /** @test */
+    public function normalize_payload_only_touches_string_email_values(): void
+    {
+        $input = [
+            'email' => null,
+            'inner' => ['email' => 12345],
+        ];
+
+        $this->assertSame($input, IdnEmail::normalizePayload($input));
+    }
+
+    /** @test */
+    public function normalize_payload_leaves_metadata_untouched(): void
+    {
+        // `metadata` is opaque application-defined data per the OpenAPI spec.
+        // The SDK must not rewrite values nested inside it, even when a caller
+        // happens to use a key named `email` for their own purposes.
+        $input = [
+            'email' => 'user@müller.de',
+            'metadata' => [
+                'email' => 'tracker+user@müller.de',
+                'nested' => ['email' => 'should-also-be-left-alone@münchen.de'],
+            ],
+        ];
+
+        $expected = [
+            'email' => 'user@xn--mller-kva.de',
+            'metadata' => [
+                'email' => 'tracker+user@müller.de',
+                'nested' => ['email' => 'should-also-be-left-alone@münchen.de'],
+            ],
+        ];
+
+        $this->assertSame($expected, IdnEmail::normalizePayload($input));
+    }
+}