Your email is confusing:
first you note that if the group has more than 5 it works ok, then you add that anything more than that is a problem.
How many people are in your group?
Have you sent to the same group before?
If you group is huge, is it possible that your mail server has a spam filter that limits the number of recipients listed on an individual message?
What version of Outlook or Express are you using?
Good luck. Mahalo,
cg