There are a few thing you may check to verify this issue, first of all depending on your notes client you may check the Preferences and Delegation tab on your configuration file. This can tell you the names of any one in the lotus notes directory that may have access to your mail database, if you do not find any entries in there none may see your mail files.
But always remember that the Notes Server’s Administrator may also have a “master” password with which he/she may have access to your mail database. This would be a serious breach in the security policy.
If you the skills you can open your mail icon and review the log