More like 3 years, and in a very niche network that was not generally deployed in companies looking for email outside of certain niches.

Absolutely, but it came several years before MIME for example for formatted emails with attachments, and other things.

SMTP is not a user interface. It’s just text from RFC821/822

This. Any engineer or manager developing Notes at the time had to be aware of X.400 and X.500, and SMTP. They decided to ignore them. However cool and useful and clumsy Notes was, it was a proprietary walled garden. Lotus and the IBM hawked it, and surely profited. But for the customer it was an expense. Eventually Notes was overtaken by Metcalf’s Law, and the fact that cheaper always wins.

That and, you know, Microsoft.