AFAIU the client itself didn't work for at least a few years. Mostly since Win 10 became the norm.

They were selling an email client that had "advanced features". Features that include:

- animation as notification. E.g. a batler appears with posh accent saying "You have mail, sir"

- you can send animation and sounds in message. When I used it, it sent embedded flash animations and sounds

- you can customize its looks

- always on top mailbox, if you remember downloaders of yore, their always on top url drop point

There were some more technically interesting features, but its selling point was cosmetics.