Lotus Notes (server), with the Domino plug in, was able to support web apps with an interactive back end database back in 1996.
Notes and Domino is a PKI based system for which every user has a certificate and can sign or encrypt messages to any user in the system. Yes they dropped the ball on extending this to the Internet and that their certs were not compatible with the internet standards. But whilst Microsoft apps were exchanging clear text passwords, Notes and Domino were exchanging encrypted tokens.
The fact that companies cheapskated on hardware, or didn't understand how to build disk arrays to support Domino, meant that they wound up turning off Full Text search, leaving users believing Notes had no search capabilities. When, in fact, it had by far the best search capabilities and that same search syntax was extended to Notes apps and also to Domino Web apps.
Even then the awareness of users as to creating a local replica and enabling Full Text searching was never there and never taught. Users just wanted to whine and companies didn't want to train users.
In 98 we were building mail systems with 1GB mailboxes and multiple replicated mailboxes to ensure failover and continuity. When other mail systems were offering 50mbytes storage to ensure stability.
The client was immensely powerful but created by a team who thought it was their way or the highway. Witness the battle royal over NOT making F5 locking the client as opposed to refresh of the view as in almost every other system. YET, even today, when I want to copy a meeting owned by someone else, in my O365 mailbox, into a new meeting of my own, keeping attendees and subject, I have to completely rebuild it. Notes is and was, right click and copy into new (calendar entry or mail or task).
In Notes you could write your own automation including the command language in the toolbars and script in database agents. You could also have extremely powerful toolbar icons by using them to call agents in your local address book, over which you had full control. In Outlook this is a nightmare from hell which gets worse as companies lock out code due to the incredibly insecure nature of Outlook code. Notes has an Execution Control list with extensive controls on what agents can do, based on a 7 layer security model.
The back end server stuff for mail routing and database have already been covered.
Notes and Domino are littered with bad choices and bad attitudes from IBM and the Dev teams. The product, itself, was transitional and still is.