Other people have good answers above, but I wanted to add a note on my experience here. I've found that when using Gmail as an outbound SMTP server for my webapp, Gmail only lets me send ~10 or so messages before responding with an anti-spam response that I have to manually step through to re-enable SMTP access. The emails I was sending were not spam, but were website "welcome" emails when users registered with my system. So, YMMV, and I wouldn't rely on Gmail for a production webapp. If you're sending email on a user's behalf, like an installed desktop app (where the user enters their own Gmail credentials), you may be okay.

此外，如果你使用Spring，这里有一个使用Gmail进行出站SMTP的工作配置:

<bean id="mailSender" class="org.springframework.mail.javamail.JavaMailSenderImpl">
    <property name="defaultEncoding" value="UTF-8"/>
    <property name="host" value="smtp.gmail.com"/>
    <property name="port" value="465"/>
    <property name="username" value="${mail.username}"/>
    <property name="password" value="${mail.password}"/>
    <property name="javaMailProperties">
        <value>
            mail.debug=true
            mail.smtp.auth=true
            mail.smtp.socketFactory.class=java.net.SocketFactory
            mail.smtp.socketFactory.fallback=false
        </value>
    </property>
</bean>

如果你想使用outlook和Javamail API，那么使用

smtp-mail.outlook.com

更多和完整的工作代码检查这个答案。

首先下载JavaMail API，并确保相关jar文件在您的类路径中。

下面是使用GMail的完整工作示例。

import java.util.*;
import javax.mail.*;
import javax.mail.internet.*;

public class Main {

    private static String USER_NAME = "*****";  // GMail user name (just the part before "@gmail.com")
    private static String PASSWORD = "********"; // GMail password
    private static String RECIPIENT = "lizard.bill@myschool.edu";

    public static void main(String[] args) {
        String from = USER_NAME;
        String pass = PASSWORD;
        String[] to = { RECIPIENT }; // list of recipient email addresses
        String subject = "Java send mail example";
        String body = "Welcome to JavaMail!";

        sendFromGMail(from, pass, to, subject, body);
    }

    private static void sendFromGMail(String from, String pass, String[] to, String subject, String body) {
        Properties props = System.getProperties();
        String host = "smtp.gmail.com";
        props.put("mail.smtp.starttls.enable", "true");
        props.put("mail.smtp.host", host);
        props.put("mail.smtp.user", from);
        props.put("mail.smtp.password", pass);
        props.put("mail.smtp.port", "587");
        props.put("mail.smtp.auth", "true");

        Session session = Session.getDefaultInstance(props);
        MimeMessage message = new MimeMessage(session);

        try {
            message.setFrom(new InternetAddress(from));
            InternetAddress[] toAddress = new InternetAddress[to.length];

            // To get the array of addresses
            for( int i = 0; i < to.length; i++ ) {
                toAddress[i] = new InternetAddress(to[i]);
            }

            for( int i = 0; i < toAddress.length; i++) {
                message.addRecipient(Message.RecipientType.TO, toAddress[i]);
            }

            message.setSubject(subject);
            message.setText(body);
            Transport transport = session.getTransport("smtp");
            transport.connect(host, from, pass);
            transport.sendMessage(message, message.getAllRecipients());
            transport.close();
        }
        catch (AddressException ae) {
            ae.printStackTrace();
        }
        catch (MessagingException me) {
            me.printStackTrace();
        }
    }
}

当然，您希望在catch块中做更多的事情，而不是像我在上面的示例代码中那样打印堆栈跟踪。(删除catch块，查看JavaMail API的哪些方法调用会抛出异常，以便更好地了解如何正确处理它们。)

感谢@jodonnel和所有回复的人。我之所以给他奖金，是因为他的回答让我找到了95%的完整答案。

这是一个使用Gmail发送电子邮件的简单类。您需要将JavaMail库添加到构建路径中，或者直接使用Maven。

import java.util.Properties;

import javax.activation.DataHandler;
import javax.activation.DataSource;
import javax.activation.FileDataSource;
import javax.mail.BodyPart;
import javax.mail.Message;
import javax.mail.MessagingException;
import javax.mail.Multipart;
import javax.mail.Session;
import javax.mail.Transport;
import javax.mail.internet.AddressException;
import javax.mail.internet.InternetAddress;
import javax.mail.internet.MimeBodyPart;
import javax.mail.internet.MimeMessage;
import javax.mail.internet.MimeMultipart;

public class GmailSender
{
    private static String protocol = "smtp";

    private String username;
    private String password;

    private Session session;
    private Message message;
    private Multipart multipart;

    public GmailSender()
    {
        this.multipart = new MimeMultipart();
    }

    public void setSender(String username, String password)
    {
        this.username = username;
        this.password = password;

        this.session = getSession();
        this.message = new MimeMessage(session);
    }

    public void addRecipient(String recipient) throws AddressException, MessagingException
    {
        message.addRecipient(Message.RecipientType.TO, new InternetAddress(recipient));
    }

    public void setSubject(String subject) throws MessagingException
    {
        message.setSubject(subject);
    }

    public void setBody(String body) throws MessagingException
    {
        BodyPart messageBodyPart = new MimeBodyPart();
        messageBodyPart.setText(body);
        multipart.addBodyPart(messageBodyPart);

        message.setContent(multipart);
    }

    public void send() throws MessagingException
    {
        Transport transport = session.getTransport(protocol);
        transport.connect(username, password);
        transport.sendMessage(message, message.getAllRecipients());

        transport.close();
    }

    public void addAttachment(String filePath) throws MessagingException
    {
        BodyPart messageBodyPart = getFileBodyPart(filePath);
        multipart.addBodyPart(messageBodyPart);

        message.setContent(multipart);
    }

    private BodyPart getFileBodyPart(String filePath) throws MessagingException
    {
        BodyPart messageBodyPart = new MimeBodyPart();
        DataSource dataSource = new FileDataSource(filePath);
        messageBodyPart.setDataHandler(new DataHandler(dataSource));
        messageBodyPart.setFileName(filePath);

        return messageBodyPart;
    }

    private Session getSession()
    {
        Properties properties = getMailServerProperties();
        Session session = Session.getDefaultInstance(properties);

        return session;
    }

    private Properties getMailServerProperties()
    {
        Properties properties = System.getProperties();
        properties.put("mail.smtp.starttls.enable", "true");
        properties.put("mail.smtp.host", protocol + ".gmail.com");
        properties.put("mail.smtp.user", username);
        properties.put("mail.smtp.password", password);
        properties.put("mail.smtp.port", "587");
        properties.put("mail.smtp.auth", "true");

        return properties;
    }
}

使用示例:

GmailSender sender = new GmailSender();
sender.setSender("myEmailNameWithout@gmail.com", "mypassword");
sender.addRecipient("recipient@somehost.com");
sender.setSubject("The subject");
sender.setBody("The body");
sender.addAttachment("TestFile.txt");
sender.send();

尽管这个问题已经结束，我想发布一个计数器解决方案，但现在使用简单Java邮件(开源JavaMail smtp包装器):

final Email email = new Email();

String host = "smtp.gmail.com";
Integer port = 587;
String from = "username";
String pass = "password";
String[] to = {"to@gmail.com"};

email.setFromAddress("", from);
email.setSubject("sending in a group");
for( int i=0; i < to.length; i++ ) {
    email.addRecipient("", to[i], RecipientType.TO);
}
email.setText("Welcome to JavaMail");

new Mailer(host, port, from, pass).sendMail(email);
// you could also still use your mail session instead
new Mailer(session).sendMail(email);

我的完整代码如下所示，运行良好:

package ripon.java.mail;
import java.util.*;
import javax.mail.*;
import javax.mail.internet.*;

public class SendEmail
{
public static void main(String [] args)
{    
    // Sender's email ID needs to be mentioned
     String from = "test@gmail.com";
     String pass ="test123";
    // Recipient's email ID needs to be mentioned.
   String to = "ripon420@yahoo.com";

   String host = "smtp.gmail.com";

   // Get system properties
   Properties properties = System.getProperties();
   // Setup mail server
   properties.put("mail.smtp.starttls.enable", "true");
   properties.put("mail.smtp.host", host);
   properties.put("mail.smtp.user", from);
   properties.put("mail.smtp.password", pass);
   properties.put("mail.smtp.port", "587");
   properties.put("mail.smtp.auth", "true");

   // Get the default Session object.
   Session session = Session.getDefaultInstance(properties);

   try{
      // Create a default MimeMessage object.
      MimeMessage message = new MimeMessage(session);

      // Set From: header field of the header.
      message.setFrom(new InternetAddress(from));

      // Set To: header field of the header.
      message.addRecipient(Message.RecipientType.TO,
                               new InternetAddress(to));

      // Set Subject: header field
      message.setSubject("This is the Subject Line!");

      // Now set the actual message
      message.setText("This is actual message");

      // Send message
      Transport transport = session.getTransport("smtp");
      transport.connect(host, from, pass);
      transport.sendMessage(message, message.getAllRecipients());
      transport.close();
      System.out.println("Sent message successfully....");
   }catch (MessagingException mex) {
      mex.printStackTrace();
   }
}
}