Outlook Distribution Lists

I am using Outlook as the mail program with Windows Live Mail as the provider.  I send out a newsletter and have a  distribution list I use.  My problem is many of the people are not receiving the "mailing" but who does not is random.  I receive back NO notification that there is a problem with sending it.  I've tried different sizes of attachments and still have the problem.  I looked at the generated list of ID's from the distribution list and everything looks OK.  If I send the exact same document to them directly (no distribution list) it works fine.  I've had them check to see I am in their contacts list, check their junk mail, etc. and can't find the problem.  Would appreciate any help you can give me.