It is a matter, which will affect a large number of clients since we all collect email contact addresses, work and personal, for ease of emailing and in fact most systems automatically add new contacts.
In the particular case the employee was employed as a publisher / editor of a magazine and conference chairman. He subsequently left employment to set up a competing business.
On his work computer he had merged several contact details from his previous employment with his personal contacts and then added his current works contacts.
He wanted to take the list with him when he left, however it was decided that because the list was a mixture of works and personal numbers he was not entitled to take it with him. If he had kept the lists separate then he could have kept his personal contacts.
Employees may wish to clarify this situation through their email policies and make it clear that unless contact lists are clearly separated then all belong to the employer.