I assume that the need for security in e-mail exchanged with financial institutions is to provide confidentiality for the message, assure message integrity, and offer non-repudiation of the message from the sender. You may want to evaluate PGP or any other product against these three requirements.
Digital signatures often provide these kinds of functions, and you should keep an eye on the development of the XML Signature, a joint specification of the Internet Engineering Task Force and World Wide Web Consortium (http://www.w3.org/Signature/). You also may want to review the ebXML technical report on security risk assessment issued in May 2001 (http://www.ebxml.org/specs/secRISK.pdf).
This was first published in January 2002