Dear X:

Upon receiving an email which states that it is your responsibility to enforce certain requirements, and said email is cc’d to your supervisor and his supervisor, it is generally not a good idea to email back, hitting reply all, and admit ignoring, and thus not enforcing, said requirements for the past five years.  Couching that admission in the language of ‘gee, we never did that before’ does not protect that admission from making you look stupid.

That is all.