There was an interesting case recently tried at the European Court regarding the right of employers to read personal emails written by their employees while at work. The ruling was that employers are in fact allowed to monitor or read any employee's emails on the basis that emails written in working hours should be work related, and hence available to be monitored by their employer.
For many people, there is increasingly a crossover between work and leisure in terms of sending and receiving emails. There is just as much chance that someone will be checking work emails at the kitchen table in the evening at home as they will be ordering something online or arranging for a home delivery while they are at work.
As with many such issues, I think the answer lies in having a common sense approach. A large number of people have unencumbered access to a computer at their workplace, which is provided essentially for work use, but as long as it isn't abused, I don't think most employees mind it being used for personal use, within reason, and specifically at lunchtime or before/after working hours.
This gives rise to yet another issue of using private mobile phones during working hours while at work, but that’s a whole other story.
However the lesson must be, don't email anything personal that could later be found to be embarrassing or compromising, just in case...