[rplnt]

Was it really HTML+JS? Because it was fast, many years back (used it just as an RSS reader though). Also, they shipped it as a standalone product[1], but I don't think it ever got updated after that.

1. http://www.opera.com/computer/mail

[gsnedders]

From memory, it was C++, much like the rest of the Opera UI. It used Quick — Opera's in-house cross-platform UI toolkit, and it died (along with the rest of the old Opera) because Quick was heavily entwined with Presto.