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What is the method of JavaMail to ensure the success of email delivery? in view of this question, this article introduces the corresponding analysis and answer in detail, hoping to help more partners who want to solve this problem to find a more simple and easy way.

People who have used JavaMail's api to send mail may wonder: how do I know if my email is successful when I call that api? In general, an open api will have a return value or status code to tell us whether the execution is successful or not. However, JavaMail does not provide such a return value.

So when we call JavaMail to send an email, we can only judge whether the email is sent successfully or not by the way of catch exception. We believe that as long as there is no exception, then the mail can be sent successfully. So let's analyze why JavaMail does not provide a return value and determine whether the status of a successful email is reliable through an exception.

The principle of sending email by JavaMail

When using JavaMail to send mail, we must provide a mail session. The procedure for creating a mail session is as follows:

Properties props = new Properties (); / / IP and port props.put ("mail.smtp.host", MAIL_SMTP_HOST) of the server that sent the mail; props.put ("mail.smtp.port", MAIL_SMTP_PORT); / / whether authentication props.put ("mail.smtp.auth", "true") is required Props.put ("mail.smtp.socketFactory.class", "javax.net.ssl.SSLSocketFactory"); Session session = Session.getDefaultInstance (props, new Authenticator () {protected PasswordAuthentication getPasswordAuthentication () {/ / username and password of the login server return new PasswordAuthentication (MAIL_SENDER_MAIL, MAIL_SENDER_PASS);}})

Analyze the code. Before creating a Session, let's create a Properties that sets several parameters: mail.smtp.host, mail.smtp.port, mail.smtp.auth, and mail.smtp.socketFactory.class. Also pass in the user name and password used to send mail when you create the Session.

The email code is as follows:

/ / 5 steps to send mail using JavaMail

/ / 1. Create session

Session session = Session.getInstance (prop)

/ / enable the debug mode of Session so that you can view the running status of the Email sent by the program

Session.setDebug (true)

/ / 2. Get the transport object through session

Transport ts = session.getTransport ()

/ / 3. Connect to the mail server with the username and password of the mailbox. When sending mail, the sender needs to submit the username and password of the mailbox to the smtp server. After the username and password have been verified, the email can be sent to the recipient normally.

Ts.connect ("smtp.sohu.com", "gacl", "mailbox password")

/ / 4. Create mail

Message message = createSimpleMail (session)

/ / 5. Send mail

Ts.sendMessage (message, message.getAllRecipients ())

/ / close the connection

Ts.close ()

There are several main steps to summarize the process of sending mail:

1. Create a Session object that contains a web link to the mail server

two。 Create a Message object that represents the content of the message

3. Create a Transport object

4. Link server

5. Send Message

6. Close the link

Because Transport is just an abstract class, the method ts.sendMessage that is called when sending Message is actually the sendMessage method of the implementation class SMTPTransport of the called Transport.

On the other hand, the sendMessage method of SMTPTransport relies on SMTP protocol to send mail.

So, when javamail uses the smtp service to send mail, when you send the message to the smtp server, you can only get the status of the queue that has been sent to smtp, but you can't get whether the mail server can be sent successfully. That is to say, you can't guarantee that the email will be successful. It depends on the content transfer of the SMTP protocol.

But if the transmission of SMTP protocol fails, it will report an error. SMTP A reliable data transfer service provided by TCP transmits mail messages from the sender's mail server to the recipient's mail server.

So we can assume that when we call JavaMail to send an email, if the program does not report an error, it means that the email was sent successfully.

SMTP working mechanism

SMTP usually has two modes of operation: sending SMTP and receiving SMTP.

The specific working mode is as follows: after receiving the mail request of the user, the sending SMTP judges whether the mail is a local mail, if it is sent directly to the mailbox of the user, otherwise, it inquires the MX record of the remote mail server to the dns, and establishes a two-way transmission channel with the remote receiving SMTP, and then the SMTP command is sent by the sending SMTP and received by the receiving SMTP, while the reply is transmitted in the opposite direction. Once the transport channel is established, the SMTP sender sends a MAIL command to indicate the sender of the message. If the SMTP recipient can receive the message, OK reply is returned. The SMTP sender then issues the RCPT command to confirm whether the message has been received. If the SMTP recipient receives it, the OK reply is returned; if it cannot be received, a reject reply is issued (but the entire message operation is not aborted), which will be repeated many times by both parties. When the recipient receives all the messages, he or she will receive a special sequence. If the receiver successfully processes the message, he can return an OK reply.

SMTP working process

Simple Mail transfer Protocol (SMTP) is a text-based e-mail transfer protocol that is used to exchange messages between mail servers on the Internet. SMTP is an application layer service, which can be adapted to all kinds of network systems.

SMTP's commands and responses are text-based, in command behavior units, with the newline character CR/LF. The response message usually has only one line, starting with a three-digit code, followed by a very short text description.

SMTP has to go through three stages: establishing a connection, sending mail and releasing the connection. The details are:

(1) establish a TCP connection.

(2) the client sends a HELO command to the server to identify the sender himself, and then the client sends a MAIL command.

(3) the server responds with OK to indicate that it is ready to receive.

(4) the client sends RCPT commands.

(5) the server side indicates whether it is willing to receive mail for the recipient.

(6) when the negotiation is over, send an email and send the input content with the command DATA.

(7) end this transmission and exit with the QUIT command.

The SMTP server records routed e-mail based on message exchange (MX) in DNS. When sending mail, the email system locates the mail server according to the address suffix of the recipient. SMTP completes the functions of editing, receiving and reading mail through the user agent program (UA), and delivers the mail to the destination through the mail transfer agent (MTA).

The answer to the question about how JavaMail ensures the success of email delivery is shared here. I hope the above content can be of some help to you. If you still have a lot of doubts to be solved, you can follow the industry information channel to learn more about it.

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