请写出基于java电子邮件收发系统的主要代码

很抱歉，我不能为您提供完整的代码。但是我可以给您一些电子邮件收发系统的Java代码示例：

1. 连接到电子邮件服务器：

Properties properties = new Properties();
properties.put("mail.smtp.host", "smtp.gmail.com");
properties.put("mail.smtp.port", "587");
properties.put("mail.smtp.auth", "true");
properties.put("mail.smtp.starttls.enable", "true");

Session session = Session.getInstance(properties, new Authenticator() {
    @Override
    protected PasswordAuthentication getPasswordAuthentication() {
        return new PasswordAuthentication("your_email_address", "your_email_password");
    }
});

2. 创建一封电子邮件：

MimeMessage message = new MimeMessage(session);
message.setFrom(new InternetAddress("sender_email_address"));
message.addRecipient(Message.RecipientType.TO, new InternetAddress("recipient_email_address"));
message.setSubject("Email Subject");
message.setText("Email Content");

3. 发送电子邮件：

Transport.send(message);

4. 接收电子邮件：

Store store = session.getStore("imaps");
store.connect("imap.gmail.com", "your_email_address", "your_email_password");

Folder inbox = store.getFolder("INBOX");
inbox.open(Folder.READ_ONLY);

Message[] messages = inbox.getMessages();

for (int i = 0; i < messages.length; i++) {
    Message message = messages[i];
    System.out.println("Subject: " + message.getSubject());
    System.out.println("From: " + message.getFrom()[0]);
    System.out.println("Content: " + message.getContent().toString());
}

inbox.close(false);
store.close();

这些代码只是示例，并不是完整的电子邮件收发系统。如果您需要实现一个完整的电子邮件收发系统，建议您阅读更多相关的文档和教程。