There’s been some jurisprudence concerning mail, probably in all countries of the world, but the cases i know the best are of course french ones.
Privacy means that your boss has no right to read your mail.
Nevertheless, some employees have been fired because it was demonstrated that, from mail, they either leaked corporate informations to concurrent companies, either wrote pejorative comments about their company, and altough mail privacy had to be infringed for such demonstrations.
Without even reading employees mails, a woman also was fired because the court recognized her rights to use the professionnal computing equipement to send SOME private mails, but only to a certain extent: in this particular case, the analysis of not the mail bodies themselves, but of their destination, had shown that more then 200 private mails per month had been sent.
And yes, if the time spent in doing this (or gaming, or more generally connecting to internet for private reasons) exceeds some amount judged by a court to be larger then a normal tolerance also depending of the function of the employee in the company, the said time is not spent at work, and therefore makes a legitimate reason to fire someone.
Not even of course speaking of the potential security or "public image " of the company involved with such activities, the web cameras in my city’s streets or the rude police controls last time i boarded a (altough domestic) flight at Orly airport also are an infringment to my privacy, but something i have to accept in the general interest of the society (and the company) i live in.
The law and internal rules of a particular company have to be written so no one can pretend not to know them, but if such precautions are taken, no one has the right to complain of an attempt to his privacy only happening to check if these rules are enforced:
I must myself abide by professionnal secret, why should i take offense, even if it does not happen, if the boss controlled my internet activities to check if i really do?
But my new boss is paranoid: she checks each hour the sales of each operator from the central server and to comment their comparison to other operators and her edicted goals, altough she is not physically present to survey the activity (when one accomplishes administrative tasks, he does not sell anything), and altough the nature of my activity sometimes leads me to refuse a particular sale: There actually is, like in a large number of companies, a control of the activity not even on the internet, but on the intranet and professionnal softwares themselves.
To take more striking exemples, do you imagine a single second someone working in a defense related activity, for a attorney, for “sensible” industrial or commercial activities… (in the end, very many people) able to surf and mail wherever he wants without being controlled?