No, an acceptable use policy does not override the law in relation to monitoring. All monitoring must be carried out in accordance with the law. The employer must take every reasonable step to advise the sender and the recipient to the email that their communications are subject to monitoring. Difficulties may arise with regards to strict enforcement policy terms. If a policy prohibits personal communications where is the line to be drawn? For example, employees may have built up personal relationships with an external party and communications between them may involve a personal element. Further is it practical, desirable or reasonable to inform in advance every external sender about the terms of your policy?
Further details can be found in the JISC Legal Interception and Monitoring Law webcast on our website at - http://www.jisclegal.ac.uk/Webcast/interception.html