Keep in mind with the file blocker that they can change mymail.pst to mymail.mail, then associate .mail files with the mail client. As long as the pst file never hits the filer, it won't stop the actual file type from getting to the filer. The native file block only looks at extension, not the file metadata to read the actual type.
Smart users will simply set the mail client up to create a file other than pst and continue to use them.
- Scott