Briefly the problem is this. Whereas in Outlook for example, we create GROUPs which is basically a list of addresses, so that we can just specify a group name in an outgoing mail and Outlook will expand it into a list of receiving email addresses. In Windows Live Mail, the feature disappeared, replaced by a thing called "Category" which is similar.

There is a bug with Category feature in Windows Live Mail. Frequently Windows Live Mail fails to expand a category properly, resulting in a number of situations ranging from empty list - no email addresses - one or some of the address on the list. When errors are spotted, I would exit and then turn on Windows Live Mail again. Eventually it will work properly, but even then, it - category expansion - may work perfectly only for most but not all categories.