simplejavamail MailerException Failed to send email reason third party error

Question

I'm trying to send a mail to the user after logging in. I'm working with Spring as the back-end. I've implemented a @GetMapping in my UserController, which calls the emailservice.mfaSenden method, sending 6 random digits.

Can someone help me out to solve this "third party problem"?

My EmailService class

package com.sep.cardgame.email;

import com.sep.cardgame.user.User;
import com.sep.cardgame.user.UserService;
import lombok.RequiredArgsConstructor;

import org.simplejavamail.api.mailer.Mailer;
import org.simplejavamail.email.EmailBuilder;
import org.simplejavamail.mailer.MailerBuilder;
import org.simplejavamail.api.email.Email;

import org.springframework.stereotype.Service;

import java.util.Random;

@Service
@RequiredArgsConstructor
public class EmailService {

    private final AppConfigs appConfigs;
    private final UserService userService;

    public void mfaSenden (User user) {
        Mailer mailer = MailerBuilder
                .withSMTPServer("localhost", 1025)
                .withDebugLogging(true)
                .async()
                .buildMailer();

        Random rand = new Random();
        int mfa = 100000 + rand.nextInt(900000);
        user.setMfaCode(mfa);
        userService.updateUser(user);

        Email email = EmailBuilder
                .startingBlank()
                .to(user.getUsername(), user.getEmail())
                 .from("noReply", "noreply@sep.com")
                 .withPlainText("Ihr 2FA Code lautet: " + mfa )
                 .buildEmail();

        mailer.sendMail(email);
        //erwer
    }
}

My UserController

@GetMapping("/login/{username}/{password}")
public ResponseEntity<User> login(@PathVariable("username") String username, @PathVariable("password") String password) {
    try{
        if(userService.findUserBySearchUsername(username) == null)
        {
            return new ResponseEntity<>(HttpStatus.NOT_FOUND);
        }
        else
        {
            User user = userService.findUserByUsername(username);

            String correctPassword = user.getPassword();
            if(correctPassword.equals(password))
            {
                if(user.isMfaActivated())
                {
                    emailService.mfaSenden(user);
                    return new ResponseEntity<>(user, HttpStatus.OK);
                }
                return new ResponseEntity<>(user, HttpStatus.OK);
            }
            else
                return new ResponseEntity<>(null, HttpStatus.UNAUTHORIZED);
        }
    }
    catch(UserNotFoundException e)
    {
        System.out.println("note: user not in db");
        return new ResponseEntity<>(null, HttpStatus.NOT_FOUND);

    } catch (Exception e)
    {
        throw new RuntimeException(e);
    }

}

pom.xml:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>3.2.5</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.sep</groupId>
    <artifactId>cardgame</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>cardgame</name>
    <description>Web-based Card Game</description>
    <properties>
        <java.version>21</java.version>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-jpa</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

        <dependency>
            <groupId>org.postgresql</groupId>
            <artifactId>postgresql</artifactId>
            <scope>runtime</scope>
        </dependency>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <optional>true</optional>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>

        <dependency>
            <groupId>org.simplejavamail</groupId>
            <artifactId>simple-java-mail</artifactId>
            <version>8.10.1</version>
        </dependency>


    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
                <configuration>
                    <excludes>
                        <exclude>
                            <groupId>org.projectlombok</groupId>
                            <artifactId>lombok</artifactId>
                        </exclude>
                    </excludes>
                </configuration>
            </plugin>
        </plugins>
    </build>

</project>

Exception:

Failed to run sendMail process
spring-boot  |
spring-boot  | org.simplejavamail.mailer.internal.MailerException: Failed to send email [Subject: 'null'], reason: Third party error