As described in "Bolt Down Your E-Mail", an end-to-end e-mail encryption architecture places the encryption keys and functionality on the user's PC. This strategy may suit an organization with only a few users who deal with sensitive information. But the benefits of end-to-end encryption may be overshadowed by the risks associated with being unable to monitor these communications.
For instance, a user could employ the encryption system to facilitate the theft or transfer of trade secrets or other confidential information. End-to-end encryption also defeats content filtering and data leak-prevention systems designed to meet regulatory requirements.
A lesser known concern is an emerging legal standard regarding failure to enforce a monitoring policy--an obvious result if you can't read encrypted messages--which may lead to serious disadvantages in civil litigation.
Under certain circumstances, your company may be prohibited by a court from retrieving, for litigation purposes, the messages sent or received by a former employee in which she discussed legal matters concerning your company with her attorney. The "work product" doctrine and the attorney-client privilege (legal rules designed to protect the confidentiality of attorneys' files and their client communications, respectively), may kick in if you fail to enforce the "personal-use ban"--the common corporate policy restricting computer use to work purposes.