/include/pear/Net/SMTP.php
Line 176: Replace $this->socket_options = $socket_options; TO
$this->_socket_options = array('ssl' => array('verify_peer_name' => false));
Line 572: Replace && extension_loaded('openssl') && isset($this->esmtp['STARTTLS']) TO
&& extension_loaded('openssl') && $this->esmtp['STARTTLS']
All problem solved for my case
OSTICKET: 1.15.3.1
PHP: v7.3.25