diff --git a/README.md b/README.md
index 307c067..118862c 100644
--- a/README.md
+++ b/README.md
@@ -28,6 +28,14 @@ $env:ADMIN_PASSWORD = "secret"
 $env:DB_URL = "jdbc:postgresql://localhost:5432/linux_distro_roguelite_promotion"
 $env:DB_USERNAME = "postgres"
 $env:DB_PASSWORD = "postgres"
+$env:TOKEN_SECRET = "<registration-token-secret>"
+$env:SMTP_HOST = "mail.fretux.ch"
+$env:SMTP_PORT = "587"
+$env:SMTP_SECURE = "0"
+$env:SMTP_USER = "no-reply@fretux.ch"
+$env:SMTP_PASS = "<smtp-password>"
+$env:MAIL_FROM = "Fretux <no-reply@fretux.ch>"
+$env:BASE_URL = "localhost:3000"
 mvn quarkus:dev
 ```
 
@@ -67,6 +75,23 @@ Die PostgreSQL-Verbindung wird ueber Umgebungsvariablen konfiguriert:
 - `DB_USERNAME`, Standard: `postgres`
 - `DB_PASSWORD`, Standard: `postgres`
 
+## Mailversand
+
+`POST /users` verschickt nach erfolgreicher Registrierung eine Bestaetigungsmail. Die E-Mail-Adresse ist Pflicht und kann wegen des eindeutigen Datenbank-Constraints nur einmal verwendet werden. Wenn der Mailversand fehlschlaegt, wird der neu angelegte Nutzer wieder entfernt und die API antwortet mit `502`.
+
+Der SMTP-Zugang wird ueber Umgebungsvariablen konfiguriert:
+
+- `TOKEN_SECRET`: Secret fuer den signierten Registrierungslink
+- `SMTP_HOST`, Standard: `mail.fretux.ch`
+- `SMTP_PORT`, Standard: `587`
+- `SMTP_SECURE`, Standard: `false`; fuer Port 587 kann `0` verwendet werden
+- `SMTP_USER`, Standard: `no-reply@fretux.ch`
+- `SMTP_PASS`: SMTP-Passwort
+- `MAIL_FROM`, Standard: `Fretux <no-reply@fretux.ch>`
+- `BASE_URL`, Standard: `http://localhost:3000`
+
+In Tests ist `quarkus.mailer.mock=true` gesetzt, damit keine echte E-Mail verschickt wird.
+
 Das Schema entspricht dem Klassendiagramm:
 
 - `users`: `id`, `username`, `email`, `newsletter_optin`, `created_at`
diff --git a/pom.xml b/pom.xml
index f5ac8c8..bc64955 100644
--- a/pom.xml
+++ b/pom.xml
@@ -57,6 +57,10 @@
             <groupId>io.quarkus</groupId>
             <artifactId>quarkus-smallrye-openapi</artifactId>
         </dependency>
+        <dependency>
+            <groupId>io.quarkus</groupId>
+            <artifactId>quarkus-mailer</artifactId>
+        </dependency>
         <dependency>
             <groupId>io.quarkus</groupId>
             <artifactId>quarkus-junit</artifactId>
diff --git a/src/main/java/dev/frederik/promotion/PromotionResource.java b/src/main/java/dev/frederik/promotion/PromotionResource.java
index d3736fb..457f922 100644
--- a/src/main/java/dev/frederik/promotion/PromotionResource.java
+++ b/src/main/java/dev/frederik/promotion/PromotionResource.java
@@ -24,10 +24,12 @@ import java.util.Map;
 @Produces(MediaType.APPLICATION_JSON)
 public class PromotionResource {
     private final DatabaseService database;
+    private final RegistrationEmailService registrationEmailService;
 
     @Inject
-    public PromotionResource(DatabaseService database) {
+    public PromotionResource(DatabaseService database, RegistrationEmailService registrationEmailService) {
         this.database = database;
+        this.registrationEmailService = registrationEmailService;
     }
 
     @GET
@@ -52,6 +54,14 @@ public class PromotionResource {
         } catch (RuntimeException exception) {
             throw mapConflict(exception, "username or email already exists.");
         }
+
+        try {
+            registrationEmailService.sendRegistrationMail(id, username, email);
+        } catch (RuntimeException exception) {
+            database.execute("DELETE FROM users WHERE id = ?", id);
+            throw new ApiException(502, "Registration email could not be sent. Please try again later.");
+        }
+
         return Response.status(Response.Status.CREATED).entity(userById(id)).build();
     }
 
diff --git a/src/main/resources/META-INF/resources/register.js b/src/main/resources/META-INF/resources/register.js
index 2ca4e93..b0737ba 100644
--- a/src/main/resources/META-INF/resources/register.js
+++ b/src/main/resources/META-INF/resources/register.js
@@ -95,7 +95,7 @@ registerForm.addEventListener("submit", async (event) => {
     );
 
     registerForm.reset();
-    setMessage("Pre-registration complete. Use Auth to unlock private game intel.");
+    setMessage("Pre-registration complete. Check your inbox, then use Auth to unlock private game intel.");
     await refreshStats();
   } catch (error) {
     setMessage(error.message, true);
diff --git a/src/main/resources/application.properties b/src/main/resources/application.properties
index 3fbe28c..89358f8 100644
--- a/src/main/resources/application.properties
+++ b/src/main/resources/application.properties
@@ -7,6 +7,16 @@ quarkus.datasource.jdbc.url=${DB_URL:jdbc:postgresql://localhost:5432/linux_dist
 quarkus.datasource.jdbc.max-size=8
 quarkus.log.category."dev.frederik.promotion".level=INFO
 
+quarkus.mailer.host=${SMTP_HOST:mail.fretux.ch}
+quarkus.mailer.port=${SMTP_PORT:587}
+quarkus.mailer.username=${SMTP_USER:no-reply@fretux.ch}
+quarkus.mailer.password=${SMTP_PASS:}
+quarkus.mailer.from=${MAIL_FROM:Fretux <no-reply@fretux.ch>}
+quarkus.mailer.tls=${SMTP_SECURE:false}
+quarkus.mailer.start-tls=REQUIRED
+registration.base-url=${BASE_URL:http://localhost:3000}
+registration.token-secret=${TOKEN_SECRET:}
+
 auth.admin.username=${ADMIN_USERNAME:admin}
 auth.admin.password=${ADMIN_PASSWORD:admin}
 
diff --git a/src/test/java/dev/frederik/promotion/PromotionIntegrationTest.java b/src/test/java/dev/frederik/promotion/PromotionIntegrationTest.java
index 6dc089e..5777aef 100644
--- a/src/test/java/dev/frederik/promotion/PromotionIntegrationTest.java
+++ b/src/test/java/dev/frederik/promotion/PromotionIntegrationTest.java
@@ -1,7 +1,10 @@
 package dev.frederik.promotion;
 
+import io.quarkus.mailer.Mail;
+import io.quarkus.mailer.MockMailbox;
 import io.quarkus.test.junit.QuarkusTest;
 import jakarta.inject.Inject;
+import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
 import javax.sql.DataSource;
@@ -21,6 +24,14 @@ class PromotionIntegrationTest {
     @Inject
     DataSource dataSource;
 
+    @Inject
+    MockMailbox mailbox;
+
+    @BeforeEach
+    void clearMailbox() {
+        mailbox.clear();
+    }
+
     @Test
     void platformsEndpointReturnsCurrentLaunchTargetsFromIsolatedDatabase() throws Exception {
         // Given: the Quarkus app started with the isolated test database and seeded launch platforms.
@@ -67,6 +78,48 @@ class PromotionIntegrationTest {
         Map<String, Object> row = findUserInDatabase(username);
         assertEquals(email, row.get("email"));
         assertEquals(Boolean.TRUE, row.get("newsletter_optin"));
+
+        List<Mail> sent = mailbox.getMailsSentTo(email);
+        assertEquals(1, sent.size());
+        assertEquals("Kernel Panic pre-registration", sent.get(0).getSubject());
+        assertTrue(sent.get(0).getText().contains("http://localhost:3000/auth.html?registration_token="));
+    }
+
+    @Test
+    void creatingSecondUserWithSameEmailIsRejected() throws Exception {
+        // Given: an email address already belongs to a pre-registered user.
+        String suffix = Long.toString(System.nanoTime());
+        String email = "unique_" + suffix + "@example.com";
+        Map<String, Object> firstPayload = Map.of(
+                "username", "first_" + suffix,
+                "email", email,
+                "newsletter_optin", false);
+
+        given()
+                .contentType("application/json")
+                .body(firstPayload)
+                .when()
+                .post("/users")
+                .then()
+                .statusCode(201);
+
+        // When: another username tries to reuse the same email address.
+        Map<String, Object> secondPayload = Map.of(
+                "username", "second_" + suffix,
+                "email", email,
+                "newsletter_optin", false);
+
+        given()
+                .contentType("application/json")
+                .body(secondPayload)
+                .when()
+                .post("/users")
+                .then()
+                .statusCode(409);
+
+        // Then: the email address is still stored only once and no second mail is sent.
+        assertEquals(1, countUsersByEmail(email));
+        assertEquals(1, mailbox.getMailsSentTo(email).size());
     }
 
     private int countCurrentPlatformsInDatabase() throws Exception {
@@ -98,4 +151,19 @@ class PromotionIntegrationTest {
             }
         }
     }
+
+    private int countUsersByEmail(String email) throws Exception {
+        try (Connection connection = dataSource.getConnection();
+             PreparedStatement statement = connection.prepareStatement("""
+                     SELECT COUNT(*)
+                     FROM users
+                     WHERE email = ?
+                     """)) {
+            statement.setString(1, email);
+            try (ResultSet resultSet = statement.executeQuery()) {
+                resultSet.next();
+                return resultSet.getInt(1);
+            }
+        }
+    }
 }
diff --git a/src/test/resources/application.properties b/src/test/resources/application.properties
index d550119..e28c5c8 100644
--- a/src/test/resources/application.properties
+++ b/src/test/resources/application.properties
@@ -4,6 +4,11 @@ quarkus.datasource.password=sa
 quarkus.datasource.jdbc.url=jdbc:h2:mem:kernel_panic_test;MODE=PostgreSQL;DATABASE_TO_LOWER=TRUE;DEFAULT_NULL_ORDERING=HIGH;DB_CLOSE_DELAY=-1
 quarkus.datasource.jdbc.max-size=4
 
+quarkus.mailer.mock=true
+quarkus.mailer.from=Fretux <no-reply@fretux.ch>
+registration.base-url=http://localhost:3000
+registration.token-secret=test-token-secret
+
 auth.admin.username=admin
 auth.admin.password=secret