https://en.wikipedia.org/wiki/Oddpost
It also felt more like an app than gmail ever has given it had the "standard" 3 panel look of most desktop email apps (same as most of them do today like Thunderbird/Outlook/Apple's Mail)
And before that Outlook had a browser version that acted like a SPA since 1997.
https://en.wikipedia.org/wiki/Outlook_on_the_web
In fact I'm pretty sure that's why Microsoft introduced XMLHttpRequest