Magento 2 SMTP could not open socket

The file to edit: vendor\magento\zendframework1\library\Zend\Mail\Protocol\Abstract.php

Find below line:

$this->_socket = @stream_socket_client($remote, $errorNum, $errorStr, self::TIMEOUT_CONNECTION);

replace it with ¹ :

$stream_context = stream_context_create(array(
 'ssl' => array(
  'verify_peer'       => false,
  'verify_peer_name'  => false,
  'allow_self_signed' => true
  )
 ));
$this->_socket = stream_socket_client(
  $remote, 
  $errorNum, 
  $errorStr, 
  self::TIMEOUT_CONNECTION, 
  STREAM_CLIENT_CONNECT, 
  $stream_context 
 );

In some cases, the server itself also blocks SMTP ports.

If you are using WHM/Cpanel, you may also need to disable “SMTP Restrictions” on WHM and in case of using such as Gmail, select “Allow less secure apps” in settings ².

Reference:

1. Source 1[↩]
2. Source 2[↩]