C++：利用E-MAIL协议内容，E-MAIL信件格式，SMTP会话过程，base64编码原理设计一个发送E-MAIL的程序

以下是一个简单的 C++ 程序示例，用于发送 E-Mail，利用 SMTP 协议、Base64 编码和 MIME 格式。

#include <iostream>
#include <string>
#include <cstdlib>
#include <cstring>
#include <sstream>
#include <fstream>
#include <winsock2.h>
#pragma comment(lib, "ws2_32.lib")

using namespace std;

// 定义 Base64 编码的转换表
const char base64_table[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

// Base64 编码函数
string base64_encode(const unsigned char* input, int length)
{
    string output;
    int i = 0, j = 0;
    unsigned char char_array_3[3], char_array_4[4];

    while (length--)
    {
        char_array_3[i++] = *(input++);
        if (i == 3)
        {
            char_array_4[0] = (char_array_3[0] & 0xfc) >> 2;
            char_array_4[1] = ((char_array_3[0] & 0x03) << 4) + ((char_array_3[1] & 0xf0) >> 4);
            char_array_4[2] = ((char_array_3[1] & 0x0f) << 2) + ((char_array_3[2] & 0xc0) >> 6);
            char_array_4[3] = char_array_3[2] & 0x3f;

            for (i = 0; (i < 4); i++)
                output += base64_table[char_array_4[i]];
            i = 0;
        }
    }

    if (i)
    {
        for (j = i; j < 3; j++)
            char_array_3[j] = '\0';

        char_array_4[0] = (char_array_3[0] & 0xfc) >> 2;
        char_array_4[1] = ((char_array_3[0] & 0x03) << 4) + ((char_array_3[1] & 0xf0) >> 4);
        char_array_4[2] = ((char_array_3[1] & 0x0f) << 2) + ((char_array_3[2] & 0xc0) >> 6);
        char_array_4[3] = char_array_3[2] & 0x3f;

        for (j = 0; (j < i + 1); j++)
            output += base64_table[char_array_4[j]];

        while ((i++ < 3))
            output += '=';
    }

    return output;
}

// MIME 格式
string create_mime(string from, string to, string subject, string content)
{
    string mime;
    mime += "From: " + from + "\r\n";
    mime += "To: " + to + "\r\n";
    mime += "Subject: " + subject + "\r\n";
    mime += "MIME-Version: 1.0\r\n";
    mime += "Content-Type: text/plain; charset=utf-8\r\n";
    mime += "Content-Transfer-Encoding: base64\r\n";
    mime += "\r\n";
    mime += base64_encode((const unsigned char*)content.c_str(), content.length());
    mime += "\r\n.\r\n";
    return mime;
}

// 发送邮件
bool send_mail(string server, string username, string password, string from, string to, string subject, string content)
{
    // 创建连接
    SOCKET sock;
    struct sockaddr_in serv_addr;
    sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_port = htons(25);
    serv_addr.sin_addr.s_addr = inet_addr(server.c_str());
    if (connect(sock, (struct sockaddr*)&serv_addr, sizeof(serv_addr)) == SOCKET_ERROR)
    {
        cerr << "Connect error!" << endl;
        return false;
    }

    // 接收信息
    char buffer[1024] = { 0 };
    recv(sock, buffer, sizeof(buffer), 0);

    // 发送 HELO 命令
    string helo = "HELO " + server + "\r\n";
    send(sock, helo.c_str(), helo.length(), 0);
    recv(sock, buffer, sizeof(buffer), 0);

    // 发送 AUTH LOGIN 命令
    string auth_login = "AUTH LOGIN\r\n";
    send(sock, auth_login.c_str(), auth_login.length(), 0);
    recv(sock, buffer, sizeof(buffer), 0);

    // 发送用户名
    string base64_username = base64_encode((const unsigned char*)username.c_str(), username.length());
    send(sock, base64_username.c_str(), base64_username.length(), 0);
    send(sock, "\r\n", 2, 0);
    recv(sock, buffer, sizeof(buffer), 0);

    // 发送密码
    string base64_password = base64_encode((const unsigned char*)password.c_str(), password.length());
    send(sock, base64_password.c_str(), base64_password.length(), 0);
    send(sock, "\r\n", 2, 0);
    recv(sock, buffer, sizeof(buffer), 0);

    // 发送 MAIL FROM 命令
    string mail_from = "MAIL FROM: <" + from + ">\r\n";
    send(sock, mail_from.c_str(), mail_from.length(), 0);
    recv(sock, buffer, sizeof(buffer), 0);

    // 发送 RCPT TO 命令
    string rcpt_to = "RCPT TO: <" + to + ">\r\n";
    send(sock, rcpt_to.c_str(), rcpt_to.length(), 0);
    recv(sock, buffer, sizeof(buffer), 0);

    // 发送 DATA 命令
    string data = "DATA\r\n";
    send(sock, data.c_str(), data.length(), 0);
    recv(sock, buffer, sizeof(buffer), 0);

    // 发送邮件内容
    string mime = create_mime(from, to, subject, content);
    send(sock, mime.c_str(), mime.length(), 0);
    recv(sock, buffer, sizeof(buffer), 0);

    // 发送 QUIT 命令
    string quit = "QUIT\r\n";
    send(sock, quit.c_str(), quit.length(), 0);
    recv(sock, buffer, sizeof(buffer), 0);

    // 关闭连接
    closesocket(sock);

    return true;
}

int main()
{
    // 设置发送邮件的参数
    string server = "smtp.gmail.com";
    string username = "your_username@gmail.com";
    string password = "your_password";
    string from = "your_username@gmail.com";
    string to = "recipient@example.com";
    string subject = "Test Mail";
    string content = "Hello World!";

    // 发送邮件
    if (send_mail(server, username, password, from, to, subject, content))
    {
        cout << "Mail sent successfully!" << endl;
    }
    else
    {
        cerr << "Failed to send mail!" << endl;
    }

    return 0;
}

请注意，在实际使用中，您需要根据您选择的邮件服务器和帐户来调整程序中的参数。此外，该程序仅适用于 Windows 平台。如果您需要在其他平台上运行，您需要对代码进行相应调整。