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.