That's what we have now, SMTP open relays are not the out of the box configuration. The problem is who owns that SMTP server. If I self host then the big providers don't want to know me. Google goes straight to spam regardless of SPF/DKIM and Microsoft flat out 550 refuses to even accept a connection. A local ISP also pre-banned me because of DigitalOcean's past lax approach to stamping out spammers on their droplets.