The only thing that can bypass SMTP authentication is relaying safe IPs (same settings page). Otherwise, only users with login details are allowed to relay mail. Note that anyone is always allowed to send to your local addresses otherwise mail wouldn't be delivered to you. It is only external email addresses that need relaying via outgoing mails that require SMTP authentication. If you send me your domain to
chris@code-crafters.com I'll run some security tests on your SMTP connection. If you want, you can also send me your remote admin login details and remote admin port and I'll have a quick look over all your main security settings.