这几天想做一个报警项目，通过esp8266简单wifi协议进行邮件发送，首先还是来看看官方示例有没有合适的。有些童鞋没有的话，可以去下载官方库 ESP Mail Client等。

示例源码如下

/**
 * This example will send the Email in
 * the html version.
 * 
 * 
 * Created by K. Suwatchai (Mobizt)
 * 
 * Email: suwatchai@outlook.com
 * 
 * Github: https://github.com/mobizt/ESP-Mail-Client
 * 
 * Copyright (c) 2021 mobizt
 *
*/

//To use send Email for Gmail to port 465 (SSL), less secure app option should be enabled. https://myaccount.google.com/lesssecureapps?pli=1

#include <Arduino.h>
#if defined(ESP32)
#include <WiFi.h>
#elif defined(ESP8266)
#include <ESP8266WiFi.h>
#endif
#include <ESP_Mail_Client.h>

#define WIFI_SSID "<ssid>"
#define WIFI_PASSWORD "<password>"

/** The smtp host name e.g. smtp.gmail.com for GMail or smtp.office365.com for Outlook or smtp.mail.yahoo.com
 * For yahoo mail, log in to your yahoo mail in web browser and generate app password by go to
 * https://login.yahoo.com/account/security/app-passwords/add/confirm?src=noSrc
 * and use the app password as password with your yahoo mail account to login.
 * The google app password signin is also available https://support.google.com/mail/answer/185833?hl=en
*/
#define SMTP_HOST "<host>"

/** The smtp port e.g. 
 * 25  or esp_mail_smtp_port_25
 * 465 or esp_mail_smtp_port_465
 * 587 or esp_mail_smtp_port_587
*/
#define SMTP_PORT esp_mail_smtp_port_587

/* The log in credentials */
#define AUTHOR_EMAIL "<email>"
#define AUTHOR_PASSWORD "<password>"

/* The SMTP Session object used for Email sending */
SMTPSession smtp;

/* Callback function to get the Email sending status */
void smtpCallback(SMTP_Status status);

void setup()
{

  Serial.begin(115200);

#if defined(ARDUINO_ARCH_SAMD)
  while (!Serial)
    ;
  Serial.println();
  Serial.println("**** Custom built WiFiNINA firmware need to be installed.****\nTo install firmware, read the instruction here, https://github.com/mobizt/ESP-Mail-Client#install-custom-built-wifinina-firmware");

#endif

  Serial.println();

  Serial.print("Connecting to AP");

  WiFi.begin(WIFI_SSID, WIFI_PASSWORD);
  while (WiFi.status() != WL_CONNECTED)
  {
    Serial.print(".");
    delay(200);
  }

  Serial.println("");
  Serial.println("WiFi connected.");
  Serial.println("IP address: ");
  Serial.println(WiFi.localIP());
  Serial.println();

  /** Enable the debug via Serial port
   * none debug or 0
   * basic debug or 1
   * 
   * Debug port can be changed via ESP_MAIL_DEFAULT_DEBUG_PORT in ESP_Mail_FS.h
  */
  smtp.debug(1);

  /* Set the callback function to get the sending results */
  smtp.callback(smtpCallback);

  /* Declare the session config data */
  ESP_Mail_Session session;

  /** ########################################################
   * Some properties of SMTPSession data and parameters pass to 
   * SMTP_Message class accept the pointer to constant char
   * i.e. const char*. 
   * 
   * You may assign a string literal to that properties or function 
   * like below example.
   *   
   * session.login.user_domain = "mydomain.net";
   * session.login.user_domain = String("mydomain.net").c_str();
   * 
   * or
   * 
   * String doman = "mydomain.net";
   * session.login.user_domain = domain.c_str();
   * 
   * And
   * 
   * String name = "Jack " + String("dawson");
   * String email = "jack_dawson" + String(123) + "@mail.com";
   * 
   * message.addRecipient(name.c_str(), email.c_str());
   * 
   * message.addHeader(String("Message-ID: <abcde.fghij@gmail.com>").c_str());
   * 
   * or
   * 
   * String header = "Message-ID: <abcde.fghij@gmail.com>";
   * message.addHeader(header.c_str());
   * 
   * ###########################################################
  */

  /* Set the session config */
  session.server.host_name = SMTP_HOST;
  session.server.port = SMTP_PORT;
  session.login.email = AUTHOR_EMAIL;
  session.login.password = AUTHOR_PASSWORD;
  session.login.user_domain = "mydomain.net";

  /* Declare the message class */
  SMTP_Message message;

  /* Set the message headers */
  message.sender.name = "ESP Mail";
  message.sender.email = AUTHOR_EMAIL;
  message.subject = "Test sending html Email";
  message.addRecipient("Admin", "####@#####_dot_com");

  String htmlMsg = "<p>This is the <span style=\"color:#ff0000;\">html text</span> message.</p><p>The message was sent via ESP device.</p>";
  message.html.content = htmlMsg.c_str();

  /** The html text message character set e.g.
   * us-ascii
   * utf-8
   * utf-7
   * The default value is utf-8
  */
  message.html.charSet = "us-ascii";

  /** The content transfer encoding e.g.
   * enc_7bit or "7bit" (not encoded)
   * enc_qp or "quoted-printable" (encoded)
   * enc_base64 or "base64" (encoded)
   * enc_binary or "binary" (not encoded)
   * enc_8bit or "8bit" (not encoded)
   * The default value is "7bit"
  */
  message.html.transfer_encoding = Content_Transfer_Encoding::enc_7bit;

  /** The message priority
   * esp_mail_smtp_priority_high or 1
   * esp_mail_smtp_priority_normal or 3
   * esp_mail_smtp_priority_low or 5
   * The default value is esp_mail_smtp_priority_low
  */
  message.priority = esp_mail_smtp_priority::esp_mail_smtp_priority_low;

  /** The Delivery Status Notifications e.g.
   * esp_mail_smtp_notify_never
   * esp_mail_smtp_notify_success
   * esp_mail_smtp_notify_failure
   * esp_mail_smtp_notify_delay
   * The default value is esp_mail_smtp_notify_never
  */
  //message.response.notify = esp_mail_smtp_notify_success | esp_mail_smtp_notify_failure | esp_mail_smtp_notify_delay;

  /* Set the custom message header */
  message.addHeader("Message-ID: <abcde.fghij@gmail.com>");

  /* Connect to server with the session config */
  if (!smtp.connect(&session))
    return;

  /* Start sending Email and close the session */
  if (!MailClient.sendMail(&smtp, &message))
    Serial.println("Error sending Email, " + smtp.errorReason());

  //to clear sending result log
  //smtp.sendingResult.clear();

  ESP_MAIL_PRINTF("Free Heap: %d\n", MailClient.getFreeHeap());
}

void loop()
{
}

/* Callback function to get the Email sending status */
void smtpCallback(SMTP_Status status)
{
  /* Print the current status */
  Serial.println(status.info());

  /* Print the sending result */
  if (status.success())
  {
    Serial.println("----------------");
    ESP_MAIL_PRINTF("Message sent success: %d\n", status.completedCount());
    ESP_MAIL_PRINTF("Message sent failled: %d\n", status.failedCount());
    Serial.println("----------------\n");
    struct tm dt;

    for (size_t i = 0; i < smtp.sendingResult.size(); i++)
    {
      /* Get the result item */
      SMTP_Result result = smtp.sendingResult.getItem(i);
      time_t ts = (time_t)result.timestamp;
      localtime_r(&ts, &dt);

      ESP_MAIL_PRINTF("Message No: %d\n", i + 1);
      ESP_MAIL_PRINTF("Status: %s\n", result.completed ? "success" : "failed");
      ESP_MAIL_PRINTF("Date/Time: %d/%d/%d %d:%d:%d\n", dt.tm_year + 1900, dt.tm_mon + 1, dt.tm_mday, dt.tm_hour, dt.tm_min, dt.tm_sec);
      ESP_MAIL_PRINTF("Recipient: %s\n", result.recipients);
      ESP_MAIL_PRINTF("Subject: %s\n", result.subject);
    }
    Serial.println("----------------\n");

    //You need to clear sending result as the memory usage will grow up as it keeps the status, timstamp and
    //pointer to const char of recipients and subject that user assigned to the SMTP_Message object.

    //Because of pointer to const char that stores instead of dynamic string, the subject and recipients value can be
    //a garbage string (pointer points to undefind location) as SMTP_Message was declared as local variable or the value changed.

    //smtp.sendingResult.clear();
  }
}

 用自己126/163邮箱试了一下，发送不成功，因为端口监视器没有截图，大致信息就是没有权限登陆进行操作，后来发现自己126/163的SMTP第三方客户端登陆授全没开，具体怎么开大家可以百度一下。（这操作挺简单的，这里发邮件只开一个IMAP/SMTP，POP3想开启也可，两个有啥区别，这里不做解释，大家自己百度)

 后面在使用官方示例，发现还是出问题，这里考虑到可能是授权码原因，重新在GitHub找了示例。我再次基础上进行修改了一下。毕竟也是开源的，因为自己项目需要，这里

#include <Arduino.h>
#if defined(ESP32)
  #include <WiFi.h>
#elif defined(ESP8266)
  #include <ESP8266WiFi.h>
#endif
#include <ESP_Mail_Client.h>
#define WIFI_SSID "ssid自己的wifi"
#define WIFI_PASSWORD "PWD 密码"
//定义


#define SMTP_HOST "smtp.126.com"
//定义端口，一般SSL 465 非SSL 25，其他的自己自行百度
#define SMTP_PORT 465
#define AUTHOR_EMAIL "自己发送邮箱账号@xxx.com"
//很多大佬给出怎么去找授权码，但没有告知授权码填哪里，这里大家注意了，授权码就是填在PASSword里
#define AUTHOR_PASSWORD "这里授权码"  //网易的是16位，其他的邮箱自己去看
#define RECIPIENT_EMAIL "接受者邮箱账号@xx.com"
  SMTPSession smtp;    
ESP_Mail_Session session;
  /* 注册回调函数，获取邮件发送状态 */
/* 定义 smtp session 对象*/


/* 获取邮件发送状态的回调函数 */
void getSmtpStatusCallback(SMTP_Status status);
/* 获取发送状态的回调函数 */
//这里大家不用看，我自己部分项目
/*int XXX = 12;
int XXXX = 13;
float XXXX;
float XXXXXX; */

void setup(){
  Serial.begin(115200);
  WiFi.begin(WIFI_SSID, WIFI_PASSWORD);
  while (WiFi.status() != WL_CONNECTED){
    delay(200);
  }
  Serial.println("WiFi 连接成功.");
  Serial.println("IP 地址: ");
  Serial.println(WiFi.localIP());
pinMode(trigPin, OUTPUT); // Sets the trigPin as an Output
  pinMode(echoPin, INPUT); // Sets the echoPin as an Input
  /* smtp开启debug，debug信息输出到串口 */

 SMTP_Message message;

}

//邮件发送模块
void alarm(){
smtp.debug(1);

 SMTP_Message message;
  /* 设置smtp 相关参数， host, port等 */
  session.server.host_name = SMTP_HOST;
  session.server.port = SMTP_PORT;
  session.login.email = AUTHOR_EMAIL;
  session.login.password = AUTHOR_PASSWORD;
  session.login.user_domain = "";

    /* 定义smtp message消息类 */
 

  /* 定义邮件消息类的名称，发件人，标题和添加收件人 */
  message.sender.name = "发送者名字";
  message.sender.email = AUTHOR_EMAIL;
  message.subject = "主题";
  message.addRecipient("接受者名字随便填", RECIPIENT_EMAIL);

  /* 设置message html 格式和内容*/

 String htmlMsg =  String(distance, DEC);
  message.html.content = htmlMsg.c_str();
  //message.html.content = htmlMsg.c_str();
  message.text.charSet = "us-ascii";
  message.html.transfer_encoding = Content_Transfer_Encoding::enc_7bit;

  /* 连接smtp服务器 */
  if (!smtp.connect(&session))
    return;

  /* 调用发送邮件函数，失败的话，获取失败信息 */
  if (!MailClient.sendMail(&smtp, &message))
    Serial.println("Fail to sending , " + smtp.errorReason());
}


void loop(){
/*这里大家也可以忽略
digitalWrite(XXX, LOW);
  delayMicroseconds(2);
  digitalWrite(XXX, HIGH);
  delayMicroseconds(2);
  digitalWrite(XXX, LOW);
  XXX= pulseIn(XXXXX, HIGH);
  Serial.println(XXXXXXX);
*/
  if (邮件发送需要触发的条件){
  alarm();
    
  }
  delay(200);
}

void getSmtpStatusCallback(SMTP_Status status){
  /* 输出邮件发送状态信息 */
  Serial.println(status.info());

  /*状态获取成功，打印状态信息 */
  if (status.success()){
    Serial.println("----------------");
    ESP_MAIL_PRINTF("邮件发送成功个数: %d\n", status.completedCount());
    ESP_MAIL_PRINTF("邮件发送失败个数: %d\n", status.failedCount());
    struct tm dt;

    for (size_t i = 0; i < smtp.sendingResult.size(); i++){
      /* 依次获取发送邮件状态 */
      SMTP_Result result = smtp.sendingResult.getItem(i);
      time_t ts = (time_t)result.timestamp;
      localtime_r(&ts, &dt);
      ESP_MAIL_PRINTF("收件人: %s邮件发送状态信息\n", result.recipients);
      ESP_MAIL_PRINTF("状态: %s\n", result.completed ? "success" : "failed");
      ESP_MAIL_PRINTF("发送时间: %d/%d/%d %d:%d:%d\n", dt.tm_year + 1900, dt.tm_mon + 1, dt.tm_mday, dt.tm_hour, dt.tm_min, dt.tm_sec);
      ESP_MAIL_PRINTF("邮件标题: %s\n", result.subject);
    }
  }
}

上面就是使用ESP8266发送邮件Send Email模块源码，里面重新补充了解释，因为项目需要，通过某些条件进行调用发邮件函数，邮件发送响应比较快，个人推荐使用，测试下很稳定。

希望可以帮助一些正在学习esp8266朋友，尽自己一份绵薄之力，感谢那些开源的大佬...