My local environment can send emails, but it doesn't work on Linux. The code is {code...} and the error message is Error: Cannot connenct to relay host smtp.163.com. I checked online and said a. Find PHP.ini and check two places. , one is allow_url_fopen, this place should be set to o...

It is possible that the port for sending emails is not open. netstat -nao Check if port 25 is open
php - You can send emails locally, but not on Linux-PHP Chinese Network Q&A

My local environment can send emails, but not on Linux
The code is
<?php

class smtp
{
 /* Public Variables */
 public $smtp_port;
 public $time_out;
 public $host_name;
 public $log_file;
 public $relay_host;
 public $debug;
 public $auth;
 public $user;
 public $pass;

 /* Private Variables */
 private $sock;

 /* Constractor */
 function smtp($relay_host = "", $smtp_port = 25, $auth = false, $user, $pass)
 {
  $this->debug = FALSE;
  $this->smtp_port = $smtp_port;
  $this->relay_host = $relay_host;
  $this->time_out = 30; //is used in fsockopen()
  #
  $this->auth = $auth;//auth
  $this->user = $user;
  $this->pass = $pass;
  #
  $this->host_name = "localhost"; //is used in HELO command
  $this->log_file = "";

  $this->sock = FALSE;
 }

 /* Main Function */
 function sendmail($to, $from, $subject = "", $body = "", $mailtype, $cc = "", $bcc = "", $additional_headers = "")
 {
  $mail_from = $this->get_address($this->strip_comment($from));
  $body = preg_replace("/(^|(\r\n))(\.)/", "\1.\3", $body);
  $header = "MIME-Version:1.0\r\n";
  if ($mailtype == "HTML") {
   $header .= "Content-Type:text/html\r\n";
  }
  $header .= "To: " . $to . "\r\n";
  if ($cc != "") {
   $header .= "Cc: " . $cc . "\r\n";
  }
  $header .= "From: $from<" . $from . ">\r\n";
  $header .= "Subject: " . $subject . "\r\n";
  $header .= $additional_headers;
  $header .= "Date: " . date("r") . "\r\n";
  $header .= "X-Mailer:By Redhat (PHP/" . phpversion() . ")\r\n";
  list($msec, $sec) = explode(" ", microtime());
  $header .= "Message-ID: <" . date("YmdHis", $sec) . "." . ($msec * 1000000) . "." . $mail_from . ">\r\n";
  $TO = explode(",", $this->strip_comment($to));

  if ($cc != "") {
   $TO = array_merge($TO, explode(",", $this->strip_comment($cc)));
  }

  if ($bcc != "") {
   $TO = array_merge($TO, explode(",", $this->strip_comment($bcc)));
  }

  $sent = TRUE;
  foreach ($TO as $rcpt_to) {
   $rcpt_to = $this->get_address($rcpt_to);
   if (!$this->smtp_sockopen($rcpt_to)) {
    $this->log_write("Error: Cannot send email to " . $rcpt_to . "\n");
    $sent = FALSE;
    continue;
   }
   if ($this->smtp_send($this->host_name, $mail_from, $rcpt_to, $header, $body)) {
    $this->log_write("E-mail has been sent to <" . $rcpt_to . ">\n");
   } else {
    $this->log_write("Error: Cannot send email to <" . $rcpt_to . ">\n");
    $sent = FALSE;
   }
   fclose($this->sock);
   $this->log_write("Disconnected from remote host\n");
  }
  echo "<br>";
  echo $header;
  return $sent;
 }

 /* Private Functions */

 function smtp_send($helo, $from, $to, $header, $body = "")
 {
  if (!$this->smtp_putcmd("HELO", $helo)) {
   return $this->smtp_error("sending HELO command");
  }
  #auth
  if ($this->auth) {
   if (!$this->smtp_putcmd("AUTH LOGIN", base64_encode($this->user))) {
    return $this->smtp_error("sending HELO command");
   }
   if (!$this->smtp_putcmd("", base64_encode($this->pass))) {
    return $this->smtp_error("sending HELO command");
   }
  }
  #
  if (!$this->smtp_putcmd("MAIL", "FROM:<" . $from . ">")) {
   return $this->smtp_error("sending MAIL FROM command");
  }

  if (!$this->smtp_putcmd("RCPT", "TO:<" . $to . ">")) {
   return $this->smtp_error("sending RCPT TO command");
  }

  if (!$this->smtp_putcmd("DATA")) {
   return $this->smtp_error("sending DATA command");
  }

  if (!$this->smtp_message($header, $body)) {
   return $this->smtp_error("sending message");
  }

  if (!$this->smtp_eom()) {
   return $this->smtp_error("sending <CR><LF>.<CR><LF> [EOM]");
  }

  if (!$this->smtp_putcmd("QUIT")) {
   return $this->smtp_error("sending QUIT command");
  }

  return TRUE;
 }

 function smtp_sockopen($address)
 {
  if ($this->relay_host == "") {
   return $this->smtp_sockopen_mx($address);
  } else {
   return $this->smtp_sockopen_relay();
  }
 }

 function smtp_sockopen_relay()
 {
  $this->log_write("Trying to " . $this->relay_host . ":" . $this->smtp_port . "\n");
  $this->sock = @fsockopen($this->relay_host, $this->smtp_port, $errno, $errstr, $this->time_out);
  if (!($this->sock && $this->smtp_ok())) {
   $this->log_write("Error: Cannot connenct to relay host " . $this->relay_host . "\n");
   $this->log_write("Error: " . $errstr . " (" . $errno . ")\n");
   return FALSE;
  }
  $this->log_write("Connected to relay host " . $this->relay_host . "\n");
  return TRUE;
 }

 function smtp_sockopen_mx($address)
 {
  $domain = preg_replace("/^.+@([^@]+)$/", "\1", $address);
  if (!@getmxrr($domain, $MXHOSTS)) {
   $this->log_write("Error: Cannot resolve MX \"" . $domain . "\"\n");
   return FALSE;
  }
  foreach ($MXHOSTS as $host) {
   $this->log_write("Trying to " . $host . ":" . $this->smtp_port . "\n");
   $this->sock = @fsockopen($host, $this->smtp_port, $errno, $errstr, $this->time_out);
   if (!($this->sock && $this->smtp_ok())) {
    $this->log_write("Warning: Cannot connect to mx host " . $host . "\n");
    $this->log_write("Error: " . $errstr . " (" . $errno . ")\n");
    continue;
   }
   $this->log_write("Connected to mx host " . $host . "\n");
   return TRUE;
  }
  $this->log_write("Error: Cannot connect to any mx hosts (" . implode(", ", $MXHOSTS) . ")\n");
  return FALSE;
 }

 function smtp_message($header, $body)
 {
  fputs($this->sock, $header . "\r\n" . $body);
  $this->smtp_debug("> " . str_replace("\r\n", "\n" . "> ", $header . "\n> " . $body . "\n> "));

  return TRUE;
 }

 function smtp_eom()
 {
  fputs($this->sock, "\r\n.\r\n");
  $this->smtp_debug(". [EOM]\n");

  return $this->smtp_ok();
 }

 function smtp_ok()
 {
  $response = str_replace("\r\n", "", fgets($this->sock, 512));
  $this->smtp_debug($response . "\n");

  if (!preg_match("/^[23]/", $response)) {
   fputs($this->sock, "QUIT\r\n");
   fgets($this->sock, 512);
   $this->log_write("Error: Remote host returned \"" . $response . "\"\n");
   return FALSE;
  }
  return TRUE;
 }

 function smtp_putcmd($cmd, $arg = "")
 {
  if ($arg != "") {
   if ($cmd == "") $cmd = $arg;
   else $cmd = $cmd . " " . $arg;
  }

  fputs($this->sock, $cmd . "\r\n");
  $this->smtp_debug("> " . $cmd . "\n");

  return $this->smtp_ok();
 }

 function smtp_error($string)
 {
  $this->log_write("Error: Error occurred while " . $string . ".\n");
  return FALSE;
 }

 function log_write($message)
 {
  $this->smtp_debug($message);

  if ($this->log_file == "") {
   return TRUE;
  }

  $message = date("M d H:i:s ") . get_current_user() . "[" . getmypid() . "]: " . $message;
  if (!@file_exists($this->log_file) || !($fp = @fopen($this->log_file, "a"))) {
   $this->smtp_debug("Warning: Cannot open log file \"" . $this->log_file . "\"\n");
   return FALSE;
  }
  flock($fp, LOCK_EX);
  fputs($fp, $message);
  fclose($fp);

  return TRUE;
 }

 function strip_comment($address)
 {
  $comment = "/\([^()]*\)/";
  while (preg_match($comment, $address)) {
   $address = preg_replace($comment, "", $address);
  }

  return $address;
 }

 function get_address($address)
 {
  $address = preg_replace("/([ \t\r\n])+/", "", $address);
  $address = preg_replace("/^.*<(.+)>.*$/", "\1", $address);

  return $address;
 }

 function smtp_debug($message)
 {
  if ($this->debug) {
   echo $message . "<br>";
  }
 }

 function get_attach_type($image_tag)
 { //

  $filedata = array();

  $img_file_con = fopen($image_tag, "r");
  unset($image_data);
  while ($tem_buffer = AddSlashes(fread($img_file_con, filesize($image_tag))))
   $image_data = $tem_buffer;
  fclose($img_file_con);

  $filedata['context'] = $image_data;
  $filedata['filename'] = basename($image_tag);
  $extension = substr($image_tag, strrpos($image_tag, "."), strlen($image_tag) - strrpos($image_tag, "."));
  switch ($extension) {
   case ".gif":
    $filedata['type'] = "image/gif";
    break;
   case ".gz":
    $filedata['type'] = "application/x-gzip";
    break;
   case ".htm":
    $filedata['type'] = "text/html";
    break;
   case ".html":
    $filedata['type'] = "text/html";
    break;
   case ".jpg":
    $filedata['type'] = "image/jpeg";
    break;
   case ".tar":
    $filedata['type'] = "application/x-tar";
    break;
   case ".txt":
    $filedata['type'] = "text/plain";
    break;
   case ".zip":
    $filedata['type'] = "application/zip";
    break;
   default:
    $filedata['type'] = "application/octet-stream";
    break;
  }
  return $filedata;
 }

} // end cla

$smtpserver = "smtp.163.com";//SMTP服务器
$smtpserverport = 25;//SMTP服务器端口
$smtpusermail = "xxx163.com";//SMTP服务器的用户邮箱
$smtpemailto = "xxx@qq.com";//发送给谁
$smtpuser = "xxx@163.com";//SMTP服务器的用户帐号
$smtppass = "xxx";//SMTP服务器的用户密码
$mailsubject = "PHP测试邮件系统";//邮件主题
$mailbody = "<h1> 这是一个测试程序</h1>";//邮件内容
$mailtype = "HTML";//邮件格式（HTML/TXT）,TXT为文本邮件
##########################################
$smtp = new smtp($smtpserver, $smtpserverport, true, $smtpuser, $smtppass);//这里面的一个true是表示使用身份验证,否则不使用身份验证.
$smtp->debug = true;//是否显示发送的调试信息
$smtp->sendmail($smtpemailto, $smtpusermail, $mailsubject, $mailbody, $mailtype);
The error message is
Error: Cannot connenct to relay host smtp.163.com
Check what is said on the Internet
a. Find PHP.ini and check two places. One is allow_url_fopen, which should be set to on; the other is to check disable_functions. If fsock and fsockopen appear later, then You need to remove these two functions, restart apache, and then you can send emails normally.
are all normal.