How to implement a simple email server using golang

With the development and popularization of the Internet, email has become an indispensable part of people's lives. This article will introduce how to use the golang programming language to implement a simple email server.

1. Environment setup

First, you need to set up the golang development environment locally. After installing golang, you need to set GOPATH. This environment variable specifies the working directory of golang. All files created in this directory are considered to be the source code of golang.

Next, install the POP3 and SMTP libraries through the following commands:

go get github.com/jordan-wright/email
go get github.com/beego/mux

The above two libraries are used to send emails and handle HTTP requests respectively.

2. Implement POP3 server

POP3 is a mail receiving protocol. Mail can be downloaded from the mail server using the POP3 protocol. In order to implement the POP3 server, we need to write the TCP server in golang. In golang, you can use the "net" package to implement TCP server development.

The following is a simple POP3 server code:

package main

import (
    "bufio"
    "fmt"
    "net"
    "strings"
)

func handlePOP3(conn net.Conn) {
    fmt.Fprintf(conn, "+OK POP3 ready\r\n")
    scanner := bufio.NewScanner(conn)
    for scanner.Scan() {
        command := scanner.Text()
        if strings.HasPrefix(command, "QUIT") {
            fmt.Fprintf(conn, "+OK Bye\r\n")
            conn.Close()
            return
        }
        fmt.Fprintf(conn, "-ERR unknown command\r\n")
    }
}

func main() {
    listener, err := net.Listen("tcp", ":110")
    if err != nil {
        fmt.Println("Error listening:", err.Error())
        return
    }
    defer listener.Close()

    fmt.Println("Listening on :110")
    for {
        conn, err := listener.Accept()
        if err != nil {
            fmt.Println("Error accepting connection:", err.Error())
            return
        }
        go handlePOP3(conn)
    }
}

The above code listens locally to port 110 (POP3 default port). When a client connects, a goroutine is started to handle the connection. . All commands received by the POP3 server are strings, so we use the Scanner provided by the bufio package to parse the commands.

In the handlePOP3 function, we first send "OK POP3 ready" to the client to indicate that the server is ready. Then, it continuously reads the commands sent by the client in a loop. If it encounters a "QUIT" command, it sends "OK Bye" to end the session and close the connection. If another unknown command is received, send "-ERR unknown command" to tell the client that the command is invalid.

3. Implement SMTP server

SMTP is a mail sending protocol. Mail can be sent to the mail server using the SMTP protocol. In order to implement the SMTP server, we need to add some code based on the POP3 server to process SMTP commands.

The following is a simple SMTP server code:

package main

import (
    "fmt"
    "net"
    "net/mail"
    "net/smtp"
)

func handlePOP3(conn net.Conn) {
    // ...
}

func handleSMTP(conn net.Conn) {
    fmt.Fprintf(conn, "220 localhost SMTP Ready\r\n")
    state := 0
    var from, to, data string
    for {
        buf := make([]byte, 1024)
        _, err := conn.Read(buf)
        if err != nil {
            fmt.Println("Error reading:", err.Error())
            return
        }
        line := string(buf)
        switch state {
        case 0:
            if line[:4] != "HELO" {
                fmt.Fprintf(conn, "500 Error: bad command sequence\r\n")
                continue
            }
            fmt.Fprintf(conn, "250 localhost\r\n")
            state = 1
        case 1:
            if line[:4] != "MAIL" {
                fmt.Fprintf(conn, "500 Error: bad command sequence\r\n")
                continue
            }
            from = line[5 : len(line)-2]
            fmt.Fprintf(conn, "250 OK\r\n")
            state = 2
        case 2:
            if line[:4] != "RCPT" {
                fmt.Fprintf(conn, "500 Error: bad command sequence\r\n")
                continue
            }
            to = line[5 : len(line)-2]
            fmt.Fprintf(conn, "250 OK\r\n")
            state = 3
        case 3:
            if line[:4] == "DATA" {
                fmt.Fprintf(conn, "354 End data with <CR><LF>.<CR><LF>\r\n")
                state = 4
            } else if line[:4] == "HELO" {
                fmt.Fprintf(conn, "250 localhost\r\n")
            } else {
                fmt.Fprintf(conn, "500 Error: bad command sequence\r\n")
            }
        case 4:
            if line[:3] == "QUIT" {
                fmt.Fprintf(conn, "221 Bye\r\n")
                conn.Close()
                return
            }
            if line == ".\r\n" {
                fmt.Fprintf(conn, "250 OK: message accepted\r\n")
                msg, _ := mail.ReadMessage(strings.NewReader(data))
                smtp.SendMail("localhost:25", nil, "test@test.com", []string{to}, []byte(data))
                state = 1
            } else {
                data += line
            }
        }
    }
}

func main() {
    // ...
    router := mux.NewRouter()
    router.HandleFunc("/pop3", handlePOP3)
    router.HandleFunc("/smtp", handleSMTP)
    http.ListenAndServe(":8080", router)
}

The above code listens locally on port 25 (SMTP default port). When a client connects, a goroutine is started to handle the connection. . All commands received by the SMTP server are also strings, so we use the Conn.Read method provided by the net package to read the commands.

In the handleSMTP function, we first send "220 localhost SMTP Ready" to the client to indicate that the SMTP server is ready. Then, maintain a state machine to handle the mail sending process:

• State 0: Process the client's HELO command and enter State 1
• State 1: Process the client's MAIL command, and Enter State 2
• State 2: Process the client's RCPT command, and enter State 3
• State 3: Wait for the client to send the DATA command, or process the HELO/QUIT command, or an error occurs, and Enter the corresponding state
• State 4: Receive the email data sent by the client (ending with "."), send the email and enter State 1

If an invalid command is received, send "500 Error: bad command sequence" tells the client that the command is invalid. If a QUIT command is received, "221 Bye" is sent to indicate the end of the session and the connection is closed. In State 4, we use the net/mail package to parse email data and the net/smtp package to send emails.

4. Test

The mailbox server implemented using the above code is just a simple example. If it is to be used in an actual production environment, many aspects of testing and optimization are required. The following is a simple SMTP client code written in python. You can use this client to send emails to our SMTP server and test whether the SMTP server is working properly:

import smtplib

server = smtplib.SMTP('localhost', 25)
server.ehlo()
server.mail('test@test.com')
server.rcpt('test1@test.com')
server.data('Subject: Test Mail\n\nThis is a test email.\n')
server.quit()

5. Summary

Introduction to this article Learn how to implement a simple email server using the golang programming language. Using golang to write SMTP/POP3 server code is easy to understand and expand. At the same time, golang's coroutine feature can support high concurrency and is very suitable for network programming development work.

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