MailItem.Categories Property (Outlook)
Returns or sets a String representing the categories assigned to the Outlook item. Read/write.
Categories is a delimited string of category names that have been assigned to an Outlook item. This property uses the character specified in the value name, sList, under HKEY_CURRENT_USER\Control Panel\International in the Windows registry, as the delimiter for multiple categories. To convert the string of category names to an array of category names, use the Microsoft Visual Basic function Split.