There are several processes for receiving mail (with attachments)
1. Receiving mail
a. determine the protocol, port, password and other parameters of the object mailbox
b. configure (protocol, port, and server) parameters through the Properties class
c. use session to store these configurations for defining environment information
d. use the Store to access the parameters in the session to connect to the mailbox
f. get inbox () through Folder (Folder is the object to operate mail)
  
2. Analyze mail
a. get mail in inbox through Message []
b. get a single Message processing object through MimeMessage, which is a Message subclass, and the instance object represents a Message
3. Save attachments
a. get complex mail processing objects through Multipart
b. get an email body of a complex email through BodyPart. BodyPart is an attribute of Multipart. Nodes such as pictures and attachments can be combined into a mixed Multipart whole through BodyPart
4. Read and process input stream

Learning links:

Application of MIME Protocol in mail

Simple example of JavaMail sending mail

Attach the personal learning demo:

public class mytest {
	public static void main(String[] args) throws Exception {

		String duankou = "110";// Port number
		String servicePath = "pop.qiye.aliyun.com";// Server address (recipient)
		String xieyi = "pop3";// Protocol (mailbox type)
		String mail = " ";// mailing address
		String password = "";
		String mydes = "H:\\ ";// Attachment saving path (file does not need to be named separately)

		receive(duankou, servicePath, xieyi, mail, password, mydes);
	}

	// Receiving mail
	public static void receive(String duankou, String servicePath, String xieyi, String mail, String password,
			String mydes) throws Exception {

		// Preparing session information to connect to the server
		Properties props = new Properties();
		props.setProperty("mail.store.protocol", xieyi); // Use pop3 protocol (specify protocol to receive mail)
		// props.setProperty("mail.transport.protocol", xieyi); / / use pop3 protocol (specify the protocol to send mail)

		props.setProperty("mail.pop3.port", duankou); // port
		props.setProperty("mail.pop3.host", servicePath); // pop3 server
		// Create Session instance object
		Session session = Session.getInstance(props);
		Store store = session.getStore(xieyi);
		// store.connect(host, user, password);
		store.connect(mail, password);

		// Get inbox
		Folder folder = store.getFolder("INBOX");//The name of a mail folder obtained through pop3 protocol can only be inbox
		folder.open(Folder.READ_WRITE); // Open inbox
        //folder.open(Folder.READ_ONLY);

		// Get all the messages in the inbox and parse them
		// Message[] messages = folder.getMessages();
		Message[] messages = folder.search(new SubjectTerm("Message subject name to access"));
		parsingMessage(messages, mydes);

	}

	// Parsing mail
	public static void parsingMessage(Message[] messages, String mydes) throws Exception {
	
		// Resolve all messages
		for (int i = 0, count = messages.length; i < count; i++) {
			MimeMessage msg = (MimeMessage) messages[i];
			msg.getMessageNumber();//What's the mail
			MimeUtility.decodeText(msg.getSubject());//Mail theme
			msg.getSentDate();//Sending time
			msg.getSize();//Message size (megabytes)
			boolean isContainerAttachment = isContainAttachment(msg);//Include attachments or not
			if (isContainerAttachment) {
				saveAttachment(msg, mydes); // Save attachments
			}
			StringBuffer content = new StringBuffer(30);//content is the body of the message
			
			//Get message body
			content.toString();//Application to plain text
			//getMailTextContent(msg, content); / / for complex body mail
		}

	}

    //Get message text content
	/*
	public static void getMailTextContent(Part part, StringBuffer content) throws Exception {
		boolean isContainTextAttach = part.getContentType().indexOf("name") > 0;
		if (part.isMimeType("text/*") && !isContainTextAttach) {//The message is plain text and the text is not empty
			content.append(part.getContent().toString());
		} else if (part.isMimeType("message/rfc822")) { //message/rfc822 Single mail body file
			getMailTextContent((Part) part.getContent(), content);
		} else if (part.isMimeType("multipart/*")) {  //multipart/*Complex mail, such as mail with attachments
			Multipart multipart = (Multipart) part.getContent();
			int partCount = multipart.getCount();
			for (int i = 0; i < partCount; i++) {
				BodyPart bodyPart = multipart.getBodyPart(i);
				getMailTextContent(bodyPart, content);
			}
		}
	}
	**/

	// Save attachments
	public static void saveAttachment(Part part, String destDir) throws Exception {
		if (part.isMimeType("multipart/*")) { // multipart / * messages with attachments
			Multipart multipart = (Multipart) part.getContent(); // Complex mail
			// Complex message contains multiple message bodies
			int partCount = multipart.getCount();
			for (int i = 0; i < partCount; i++) {
				// Get one of the complex body messages
				BodyPart bodyPart = multipart.getBodyPart(i);
				// A mail body may also be a complex body composed of multiple mail bodies
				String disp = bodyPart.getDisposition();
				if (disp != null && (disp.equalsIgnoreCase(Part.ATTACHMENT) || disp.equalsIgnoreCase(Part.INLINE))) { // Indicates that the msg has content, the file has come down or can be opened
					InputStream is = bodyPart.getInputStream();
					saveFile(is, destDir, MimeUtility.decodeText(bodyPart.getFileName())); //MimeUtility.decodeText file decoding
				} else if (bodyPart.isMimeType("multipart/*")) {
					saveAttachment(bodyPart, destDir);
				} else {
					String contentType = bodyPart.getContentType();
					if (contentType.indexOf("name") != -1 || contentType.indexOf("application") != -1) {
						saveFile(bodyPart.getInputStream(), destDir, MimeUtility.decodeText(bodyPart.getFileName())); //MimeUtility.decodeText file decoding
					}
				}
			}
		} else if (part.isMimeType("message/rfc822")) {
			saveAttachment((Part) part.getContent(), destDir);// getContent is to get and display the message content
		}

	}


	// Include attachments or not
	public static boolean isContainAttachment(Part part) throws Exception {
		boolean flag = false;
		if (part.isMimeType("multipart/*")) {
			MimeMultipart multipart = (MimeMultipart) part.getContent();
			int partCount = multipart.getCount();
			for (int i = 0; i < partCount; i++) {
				BodyPart bodyPart = multipart.getBodyPart(i);
				String disp = bodyPart.getDisposition();
				if (disp != null && (disp.equalsIgnoreCase(Part.ATTACHMENT) || disp.equalsIgnoreCase(Part.INLINE))) {
					flag = true;
				} else if (bodyPart.isMimeType("multipart/*")) {
					flag = isContainAttachment(bodyPart);
				} else {
					String contentType = bodyPart.getContentType();
					if (contentType.indexOf("application") != -1) {
						flag = true;
					}

					if (contentType.indexOf("name") != -1) {
						flag = true;
					}
				}
				if (flag)
					break;
			}
		} else if (part.isMimeType("message/rfc822")) {
			flag = isContainAttachment((Part) part.getContent());
		}
		return flag;
	}

    	// Read input is saved to the specified location in turn
	public static void saveFile(InputStream is, String destDir, String fileName) throws Exception {
		BufferedInputStream bis = new BufferedInputStream(is);
		BufferedOutputStream bos = new BufferedOutputStream(new FileOutputStream(new File(destDir + fileName)));
		int len = -1;
		while ((len = bis.read()) != -1) {
			bos.write(len);
			bos.flush();
		}
		bos.close();
		bis.close();

	}



}