diff -urN ../2.6.4+0.orig/src/backend/imap/config.php ./src/backend/imap/config.php
--- ../2.6.4+0.orig/src/backend/imap/config.php	2021-06-25 11:55:30.000000000 +0300
+++ ./src/backend/imap/config.php	2023-06-18 21:49:00.614076000 +0300
@@ -150,6 +150,7 @@
 // FIELDS: columns in the query
 // FROM: string that will be the from, replacing the field names with the values
 define('IMAP_FROM_LDAP_SERVER_URI', 'ldap://127.0.0.1:389/');
+//define('IMAP_FROM_LDAP_START_TLS', true);
 define('IMAP_FROM_LDAP_USER', 'cn=zpush,ou=servers,dc=zpush,dc=org');
 define('IMAP_FROM_LDAP_PASSWORD', 'password');
 define('IMAP_FROM_LDAP_BASE', 'dc=zpush,dc=org');
diff -urN ../2.6.4+0.orig/src/backend/imap/user_identity.php ./src/backend/imap/user_identity.php
--- ../2.6.4+0.orig/src/backend/imap/user_identity.php	2021-06-25 11:55:30.000000000 +0300
+++ ./src/backend/imap/user_identity.php	2023-06-18 21:52:50.037496000 +0300
@@ -138,6 +138,26 @@
                 ZLog::Write(LOGLEVEL_DEBUG, sprintf("BackendIMAP->getIdentityFromLdap() - Connected to LDAP"));
                 ldap_set_option($ldap_conn, LDAP_OPT_PROTOCOL_VERSION, 3);
                 ldap_set_option($ldap_conn, LDAP_OPT_REFERRALS, 0);
+
+                if (defined('IMAP_FROM_LDAP_START_TLS') and (IMAP_FROM_LDAP_START_TLS === true)) {
+                    ZLog::Write(LOGLEVEL_DEBUG, "BackendIMAP->getIdentityFromLdap() - Try to start TLS");
+                    if (! @ldap_start_tls($ldap_conn)) {
+                        $ldap_errno = ldap_errno($ldap_conn);
+                        $ldap_error = ldap_error($ldap_conn);
+                        $ldap_conn = false;
+                        $message = sprintf(
+                            "BackendIMAP->getIdentityFromLdap() - Could not start TLS session with LDAP server %s (error %d: %s)",
+                            IMAP_FROM_LDAP_SERVER_URI, $ldap_errno, $ldap_error
+                        );
+                        if (defined('IMAP_FROM_LDAP_SERVICE_UNAVAILABLE_ON_ERROR') and (IMAP_FROM_LDAP_SERVICE_UNAVAILABLE_ON_ERROR === true)) {
+                            ZLog::Write(LOGLEVEL_WARN, $message);
+                            $ret_value = '';
+                        } else {
+                            ZLog::Write(LOGLEVEL_DEBUG, $message);
+                        }
+                    }
+                }
+
                 $ldap_bind = ldap_bind($ldap_conn, IMAP_FROM_LDAP_USER, IMAP_FROM_LDAP_PASSWORD);
 
                 if ($ldap_bind) {