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.