Question about Outlook Express

Since the fall 2013 purchase of a new Dell running Windows 7 and Microsoft Office 2013, our office migrated from Outlook Express to Windows Live Mail.  We have approx. 100 employees who we contact via email. We have them set up in two categories - new hires and returning staff.  Upon initial set up, the categories work fine when we send emails, but then over a period of several days or weeks, selecting a category to send an email results in only some, random individuals in the group receiving it.  Belatedly, If we click on the plus to expand the sent to list, we can see that only half or less of the addresses in the category are included in the sent email.  This creates issues in running our business effectively.  If I rebuild the category, it works for a while, then randomly stops again.  I have seen the same issue mentioned in other blogs and forums going back through 2012 and 2013 with no solutions.  Running repairs and reinstalling are ineffective. Is this an inherent problem with Windows Live?