Re: first rule of email admin
Probably the second rule is something like "even though I have no interest in reading your mail, each and every host it goes through does have the capability of reading it: assume that I'm the exception among these admins and if you want privacy, encrypt the mail or don't use email at all".
Not using email at all would have been the smart thing to do in this case, since the recipient metadata is still in the clear. But then, the sender probably wasn't the sharpest tool in the box and no amount of explaining would have led him to do the sensible thing.
Best course for this admin would have been to refuse to scan the emails in the first place. Or only set up filtering with the policy that all misaddressed mail will go directly to a public (office-wide) noticeboard. Either that, or refuse to look at the content and base redirections solely on the To: field. I prefer the more dramatic option, though.