It's difficult for many people to run their own mail servers (really fun convincing all the other servers in the world that you aren't a spammer), and that only works if all parties in the conversation do it. As for mitigating it, I wouldn't expect deleting my mail on someone else's server to make it unavailable to anyone but me.
End-to-end encryption is the answer. Even a simple trust-on-first-use scheme makes the barriers to eavesdropping much higher (i.e. existent). It still has network effect problems though - most people use Android email clients that don't support encryption (presumably iPeople are in a worse boat, but that's their choice), most people use webmail for some reason, etc. People at large seem to be achieving the first glimmers of lucidity on the importance of privacy in The Information Age, so hopefully that can be overcome. The only downside once more people are using it is that it destroys the revenue model of all free email providers (i.e. reading your mail).