I'm fan of split design for web browsing for security reasons - trusted web sites can be accessed via http proxy from pc clients, the rest of web sites via some remote desktop technology from specific servers only residing in dmz.
I'm not sure how to make some good preventive design for mail, especially to be protected against malicious attachments (no, antivirus is not enough). Today a pdf can contain code which while the pdf is printed can hack a printer for example.
What are example designs for some secure mail handling?
Do exist some good documents about this topic?