Again, only between users with skiff emails, which is a negligible portion of all emails. This needs to be made clear as it is very misleading to the average user who probably thinks all emails are end to end encrypted.
As a simple demonstration, even if client side code is perfectly secure, an adversary with server control can simply log all emails passing through the server and instantly have access to all new user emails that way. This means users have to trust the server, contradicting any notion of E2EE.