Ok. Scratch that idea. Your solution lies with Mimecast. They use an LDAP link to intercept all mail both internal and external. You are given full control as to what how when and where. Those are the basics that you needed right. The security is tighter than that of a bank. In fact banks use them. I signed up about 6 months ago and it solved my problems like crazy. Trust me on this one. The other tools that comes with are amazing. Mail wrap, remote mail and back ups are essential to any IT head. Let me know how it goes.