Do you really think your shared host drops blocks when you delete them from your virtual disk, and do you really think that requests to mlock memory with crypto keys are really honored? Maybe if you have a dedicated box, but not if you are using a virutal host. (Have you ever physically seen "your own" mail server? If not, why do you trust it?)

Also consider what happens to unencrypted email you send or receive: any upstream servers can be subpoenaed, as can the person on the other end. Encrypted mail only works when it's encrypted end-to-end and your adversary cannot seize the sending or receiving computer. I imagine that people who encrypt their received email end up in court because their sender was not so careful. Do you trust every person that will ever mail you to keep your secrets safe? Why?

Ultimately, if the government is your enemy, you need to take a lot more precautions than "don't use Gmail".