This patch can solve the issue:

diff -ru osticket.orig/include/pear/Net/SMTP.php osticket.fixed/include/pear/Net/SMTP.php
--- osticket.orig/include/pear/Net/SMTP.php	2020-12-07 19:23:52.000000000 +0100
+++ osticket.fixed/include/pear/Net/SMTP.php	2021-02-13 09:58:08.821455158 +0100
@@ -569,7 +569,7 @@
          * STARTTLS extension, and aren't already connected over a secure
          * (SSL) socket connection. */
         if ($tls && version_compare(PHP_VERSION, '5.1.0', '>=')
-            && extension_loaded('openssl') && isset($this->esmtp['STARTTLS'])
+            && extension_loaded('openssl') && $this->esmtp['STARTTLS']
             && strncasecmp($this->host, 'ssl://', 6) !== 0
         ) {
             /* Start the TLS connection attempt. */

Regards,
Anthony Ananich
https colon slash slash ananich dot pro