We have two laptops. On each one we have two distribution groups that contain over 50 email addresses each. On one laptop we can send a message to either group of 50 with no problem. On the other laptop the emails can not be sent to a group unless there are only about 15 names on the list. Why is this happening.
Are you using only the webmail interface with a browser on each machine? Or are you using a client like Outlook or Thunderbird? In either case, on the one with the problem, you need to check each contact in the group to make sure there are no typos, missing characters, spaces and so on. If only one is wrong it can keep the entire group from being sent.