[mike-cardwell]

> Don't these typically require access to the private key though?

Yes, but not on the server. The key is typically stored encrypted in the browser storage. Never hits the server.

But there is still the problem where the server could send "bad" javascript which copies the key and uploads to the server.

However, if it's my server and I'm running the webmail, I might be ok with that. And if the server is being run by somebody I trust, I might still be ok with making that decision.

And even if I don't want to add my own private key, it would still be nice if the webmail could verify messages signed by other people. There's nothing risky about that.