Ok, you do realize all data in and out of a BB is encrypted so the security level is way beyond any other cell phone. To add any level of additional security both ends must be able to decode the email. Generally using full PGP requires an application running on a server to forward the data to a trusted receiver. Symantec does offer a full solution but does not specifically indicate they require a BES to support the PGP . It might be worth checking to see the requirements and limits and maybe it will work for you.

One application you might want to check is Ewallet.