Send invite emails asynchronously

backend/src/main/java/backend/fullstack/Main.java

@@ -2,8 +2,10 @@
 
 import org.springframework.boot.SpringApplication;
 import org.springframework.boot.autoconfigure.SpringBootApplication;
+import org.springframework.scheduling.annotation.EnableAsync;
 
 @SpringBootApplication
+@EnableAsync
 public class Main {
     public static void main(String[] args) {
         SpringApplication.run(Main.class, args);

backend/src/main/java/backend/fullstack/auth/invite/AsyncMailSender.java

@@ -0,0 +1,37 @@
+package backend.fullstack.auth.invite;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import org.springframework.mail.SimpleMailMessage;
+import org.springframework.mail.javamail.JavaMailSender;
+import org.springframework.scheduling.annotation.Async;
+import org.springframework.stereotype.Component;
+
+import java.util.Optional;
+
+/**
+ * Sends emails on a background thread so SMTP hangs never block HTTP responses.
+ */
+@Component
+public class AsyncMailSender {
+
+    private static final Logger logger = LoggerFactory.getLogger(AsyncMailSender.class);
+
+    private final Optional<JavaMailSender> mailSender;
+
+    public AsyncMailSender(Optional<JavaMailSender> mailSender) {
+        this.mailSender = mailSender;
+    }
+
+    @Async
+    public void send(SimpleMailMessage message) {
+        if (mailSender.isEmpty()) {
+            return;
+        }
+        try {
+            mailSender.get().send(message);
+        } catch (Exception ex) {
+            logger.error("Failed to send email to {}: {}", message.getTo(), ex.getMessage());
+        }
+    }
+}

backend/src/main/java/backend/fullstack/auth/invite/UserInviteService.java

@@ -35,6 +35,7 @@ public class UserInviteService {
     private final UserRepository userRepository;
     private final PasswordEncoder passwordEncoder;
     private final Optional<JavaMailSender> mailSender;
+    private final AsyncMailSender asyncMailSender;
     private final InviteProperties inviteProperties;
     private final SecureRandom secureRandom = new SecureRandom();
 
@@ -43,12 +44,14 @@ public UserInviteService(
             UserRepository userRepository,
             PasswordEncoder passwordEncoder,
             Optional<JavaMailSender> mailSender,
+            AsyncMailSender asyncMailSender,
             InviteProperties inviteProperties
     ) {
         this.userInviteTokenRepository = userInviteTokenRepository;
         this.userRepository = userRepository;
         this.passwordEncoder = passwordEncoder;
         this.mailSender = mailSender;
+        this.asyncMailSender = asyncMailSender;
         this.inviteProperties = inviteProperties;
     }
 
@@ -113,13 +116,8 @@ private void sendInviteEmail(User user, String token) {
             return;
         }
 
-        try {
-            mailSender.get().send(message);
-            logger.info("Invite email sent to userId={} email={}", user.getId(), user.getEmail());
-        } catch (Exception ex) {
-            logger.error("Failed to send invite email to userId={} email={}: {}",
-                    user.getId(), user.getEmail(), ex.getMessage());
-        }
+        logger.info("Sending invite email to userId={} email={}", user.getId(), user.getEmail());
+        asyncMailSender.send(message);
     }
 
     /**