You may consider e-mail a secure tool. I am not very sorry to disapoint you - it is not. You may continue to hide your head in the sand, or reconsider your use of e-mail. Enterprise wide, policybased encryption is one road to salvation. An alternative is PGP and its like - but this means your users actually has to encrypt the messages themselves.
I suggest a mix of awareness of the threat, a set of policies to handle secure data, and then tools to incorporate the tools.
PrivaWall looks interesting,