This position could be justified under the argument that the employer is the one who sets up the rules in a working place, therefore employees should act in conformity to same, whether they accept to work for that employer. Different is the analysis that deserves a case where the employer searches the employee's e-mails without any previous notice. Many campaigners believe employers' power to scan e-mails directly contravene the Human Rights and Data Protection Acts, which state individuals have a right to privacy at work. Privacy facing property rights not an easy task. A suitable way of handling this matter might be to adopt something similar to what Microsoft has done, i.e. a policy promoting the reasonable and responsible personal use of corporate e-mail.
People are old and wise enough to make their own decisions on what they send and read.
Although I consider the position of Microsoft may be a good alternative, I cannot deny that the words reasonable and responsible have a broad meaning and may lead to many different interpretations. It's upon the employee to show that he is using the corporate e-mail service in a reasonable way. There are a few laws challenging the corporate view that any information sent or received by an employee's computer is company property. Employers should strike a balance between privacy and surveillance but there is little doubt that unscrupulous employers could abuse the rules.