Assume that the success rate on oral instructions is about 60%. In other words only about 6 out of 10 oral instructions are ever carried out, the younger the staff the less the chance an oral instruction will be carried out. If you really want it done the instruction must be given in writing.
Spam on email is ruining the efficacy of email as a means of communication. Spam filters and the fact that the eye looks for what emails to delete means that many emails are not getting through – back up email instructions with a written document.