A person in my company has returned from vacation and to his surprise, some of his messages, dating from the period he was away, lost their unread mark.
Would there be another reason (technical) for this situation or did someone inside our company (maybe outside too) is hacking his mailbox?
It sounds to me like someone did read some of this person's email messages. Most likely, they were using the person ID when they did this. I do not know how this would happen to random messages by accident (although anything is possible with software).
For more information, see Tech Note #169936 in the Notes Knowledge Base. Go to http://www.lotus.com/home.nsf/welcome/support, enter 169936 into the search box and read this technote. It tells you more than you want to know about unread marks.
This was first published in August 2001