I placed a bug RFE for this, but was also hoping to "stir up" some interest here in these forums.
Since Zimbra has made a strategic partnership with ProofPoint to provide Encryption services, I believe it would be in VMWare/Zimbra's interest to provide direct integration with ProofPoint's encryption software. That is, work with ProofPoint to directly integrate their product into the server install, much like with A&D. This provides for a quick, out-of-the-box solution without the need to setup additional software or appliances, it would simply work on the same box that Zimbra runs on. Furthermore, I could also see the benefit of integrating the management side of ProofPoint's encryption software with Zimbra. This will then provide for a single point of manageability.
Bottom line: Integration of Zimbra + ProofPoint will make for much easier management and less topological outlay. For those with multiple servers in a cluster, the encryption could be installed on all devices with an MTA and still be beneficial.
Take a look at: Bug 65987 – Direct Integration of ProofPoint Encryption (Server-Side) for more information and to cast your vote.
Thanks for your consideration of this RFE.