出处： https://studygolang.com/articles/14256

• 还没有验证过，代码验证

package main

import (
    "fmt"
    "net"
    "os"
)

type Mail struct {
    clientName string
    from       string
    to         string
    data       string
    reciveData bool
}

func main() {

    listen, err := net.ListenTCP("tcp", &net.TCPAddr{net.ParseIP("127.0.0.1"), 25, ""})
    if err != nil {
        fmt.Println("监听端口失败:", err.Error())
        return
    }
    defer listen.Close()
    recive(listen)

}

func recive(listen *net.TCPListener) {

    for {
        conn, err := listen.AcceptTCP() //接收连接
        defer conn.Close()
        if err != nil {
            fmt.Println("客户端连接异常", err.Error()) //客户端连接异常
            continue
        }
        conn.Write([]byte("220 GoMailSever "))
        fmt.Println("新客户端", conn.RemoteAddr().String())
        go func() {

            tmpBuffer := make([]byte, 0)           //创建待处理字节切片
            readerChannel := make(chan []byte, 16) //创建channel通信管道
            _mail := &Mail{}
            go reader(readerChannel, conn, _mail) //channel信息接收端
            data := make([]byte, 1024)            //创建输出缓冲切片

            for {
                i, err := conn.Read(data) //从Socket读取数据
                if err != nil {
                    fmt.Println("读取发生错误", err.Error()) //读取失败发生错误
                    return
                }
                tmpBuffer = DepackMail(append(tmpBuffer, data[:i]...), readerChannel) //用自定义协议处理字节

                //fmt.Println(data, string(data))
            }

        }()

    }
}
func reader(readerChannel chan []byte, conn *net.TCPConn, _mail *Mail) {

    for {
        select {
        case data := <-readerChannel:
            fmt.Println(string(data[0:4]))
            switch string(data[0:4]) {
            case "HELO":
                _mail.clientName = string(data[4+2:])
                conn.Write([]byte("250 Give Me Mail"))
                break
            case "MAIL":
                _mail.from = string(data[4+2:])
                conn.Write([]byte("250 Give I am fine"))
                break
            case "RCPT":
                _mail.to = string(data[4+2:])
                conn.Write([]byte("250 Give I am fine"))
                break
            case "DATA":
                conn.Write([]byte("354 Give I am fine"))
                break
            case "QUIT":
                conn.Write([]byte("221 ByeBye"))
                file, _ := os.Create("./mail/3.eml")
                file.WriteString(_mail.data)
                file.Close()

                fmt.Println(_mail)
                break
            default:
                _mail.data = _mail.data + string(data)
                fmt.Println(string(_mail.data[len(_mail.data)-1:]))
                if string(_mail.data[len(_mail.data)-1:]) == "." {
                    fmt.Println("111")
                    conn.Write([]byte("250 Give I am fine"))
                }

                break
            }

        }
    }
}
func DepackMail(buffer []byte, readerChannel chan []byte) []byte {
    length := len(buffer)

    var i int

    for i = 0; i < length; {
        if length < i+4 { //2位为尾标记
            break
        }
        var sIndex int
        for dump := i; dump < length-1; dump++ {
            if buffer[dump] == byte(13) && buffer[dump+1] == byte(10) {

                sIndex = dump
            }
        }
        if sIndex <= 0 {
            break
        }
        data := buffer[i:sIndex]

        readerChannel <- data //向管道发送数据
        i = sIndex + 2
    }

    if i == length { //全部处理完毕返回空包
        return make([]byte, 0)
    }
    return buffer[i:] //未处理完毕返回 剩余包
}