Yes, it is likely that employers tap their employee's electronic messages, because they want to know what their employees are doing. Employers want to know if their employees are wasting time at work, and they also want to know if their employees are doing anything illegal. If it is their email, it is fair to look.
Yes, I believe it is very likely that employers tape (I hope the question meant tape) the electronic messages of employees. Employers can be liable for things employees say or do while on company property or on company business, so it would be something they need to protect themselves. Most companies I have dealt with make no secret of the fact that they monitor phone conversations and emails and have you sign a document acknowledging that you are aware of the practice.
Yes, I think it is more than likely that employers tap their employee's electronic messages. If you are using the companies Internet connection or electronic devices it is their right to monitor what you are doing while are utilizing their equipment and their time. If you do not want someone to see it do not use the company's equipment.
I believe that the majority of employers with the capabilities do tap thier employees electronic messages. If they have the means of doing so, employees will look into what the employees are sending to see if they are being productive or not. Looking to see what messages employees are sending, employers can see if they personal or work related. Being able to see if an employee is engaging in personal messaging during work hours allows an employer to see if they are working or goofing off.
It is not only likely, it is almost a certainty that employers tap their employee's electronic messages. Already, many employee contracts allow all their computer usage to be monitored by their employer and many employers actively block connections that attempt to mask employee activity. They would not go to these lengths if they did not intend to monitor electronic messages.