You should not encurage users to send encrypted e-mails only with sensitive data.
Anyone who survey their computer will see a pattern and will be encouraged to try to decrypt them! And with Amazon server farm you can rent at low costs anyone can brute-force them in little to no-time (think how fast NSA will do it)... and have access to ALL YOUR CRITICAL INFO.
On the other hand, when you encrypt all your mails and they try to decode them only to find plain talking... they will loose interest and chances are that your sensitive info will pass un-noticed.
I got tired of google reading all of my emails and got hushmail instead. They dont read your emails and you can encrypt them if you choose to with the whole security password thing mentioned in this article.
We are tackling the email encryption problem on a different level: we are writing a completely new protocol where public keys are distributed automatically via a DHT and the mails are always encrypted without user intervention.
The system is peer to peer (doesn't need any central server) and uses the Distributed Hash Table to distribute the keys & the list of available mails. When the recipient is offline then the mails are stored temporarily on other peers and then transferred using a bittorrent-like protocol when the recipient is back online.
More info here: http/igg.me/at/flowingmail/x/3978171