Doesn't Gmail support SSL? My email provider provides support for SSL connections (via HTTP, LDAP, & POP).
If that's the case then it would be extremely difficult (you will need to "fake" the server's host keys). Furthermore, the ethics of such a practice is questionable. For which case I would side on blocking outside emails altogether rather than get into a situation where you have to justify sniffing someone's personal bank transactions, e.g. -a
