SMTP Error (550): Failed to set sender “firstname.lastname@example.org” (Access denied – Invalid HELO name (See RFC2821 4.1.3)).
You should add smtp helo name in the mail configuration. Find the file config.inc.php and edit it
add the line.
$config['smtp_helo_host'] = 'localhost';
Now login to your mail client and test the outgoing mail by sending one.