That only works for internal communications. Once it leaves Google's servers, you lose all control. I don't know the specifics here, but the only ways to guarantee that an email server somewhere isn't caching your emails (and I don't trust Google to not cache them either) is to either encrypt them (GPG) or require hitting your server to read the email (potentially what Google is doing), and that doesn't prevent the user from copying it (but at least you can know _who _ copied it or let it be copied).

I don't know how external access works, so maybe they're doing more than they say they are, but I don't trust my coworkers, I shouldn't trust Google either. Client-side encryption is the only acceptable solution IMO.