What is needed for S/MIME remailer ?

The other day I was wondering what it would take to make an anonymous remailer for an S/MIME client. Implementing S/MIME from scratch is a lot of work given the lack of a public domain implementation, but it seems that a remailer could be built on top of a commercial email product that supported S/MIME. The question is: What features should a commercial S/MIME email product have to make it easy to support a remailer? Are there a small set of command line options that would greatly simplify a remailer implementation? --Bob Baldwin