POP3 isn't clever, especially to users of multiple machines. POP3 was perfectly fine for most users in 1984, but there's a reason why IMAP was even developed let alone widely adopted. It would only still store emails on the server, the deleting on retrieval part is optional - and due to the issues this can cause should be disabled - in cases where users are unlikely to frequently check and or maintain their inbox, then it's unlikely to provide for an effective solution.
Mainstream email clients are not the main reason email services tend to fall prey. It's most commonly unsafe practices taken by the user that literally hand over access. No client side deficit should allow to mitigate serverside security save for allowing third parties access to the authentication details.
TOR, not taking into account it not being email, is possibly best something avoided if possible for a variety of reasons. A VPN - also not email - is a more sensible solution to the problem most people think TOR is intended to solve, and you would want exit nodes in more places than Stockholm to provide a usable service.
I'm not aware of any open source P2P email solutions. Generally tunneling data that may contain "sensitive information" is most sensible to transfer through the smallest number of random third parties as possible. Preferably 0.
Skype, also not email and providing for third-party on-demand transparent forwarding of the desktop, filing system, keyboard, mouse movements, gestures and clicks, microphone, camera, and a lovely list of exploits stored server-side to unload, is something I hope you included to inject some humour, because I laughed when I read it. "solutions" that "monitor" use and collect conversation data should not be considered options.
Double 128-bit crypto, on what exactly? What cipher(s)? 128-bit sounds alarmingly low, too....
QQ I have little experience with, but considering it's popularity in china, and the way their government hasn't attempted to outlaw it, it's highly likely to introduce as many if not more defects than skype and additionally hand over all conversations to the chinese government.
IRC might be old, but is also another example this not reducing from it's effectiveness, however, it attempts to solve a slightly different problem to email. It's also commonly heavily targeted for a wide range of abuses, and many ircd's are not that great at standing up to some abuses. XMPP would be a "better" solution doing everything IRC can do, and bringing more features to the table in the process. It would be able to be trivially implimented with a web portal, or standalone clients on user's devices. Unlike IRC you are required to authenticate before it even lets you on the server, and web-based implimentations of IRC are traditionally somewhat lacking.