Sure, but Notes was built for a world where ccMail, MS Mail, and GroupWise were the main workgroup email systems.

Notes followed a similar pattern, with a proprietary "post office" system with optional gateways to larger x.400 / SMTP / UUCP / OfficeVision networks for those who needed them.

Going SMTP (or even x.400)-native from the start might have been a better choice in the long run, but that's only clear in retrospect - it certainly wasn't obvious at the time.