You can view the memberships of Global Address Lists or created distribution lists so that you know to whom you are sending a message. 

Viewing memberships of a list

To view memberships of a list,

  1. compose a new message as usual.
  2. Add a list to the To:, Cc:, or Bcc: fields as usual.
  3. In the Outlook client, double-click the list. In the Outlook Web App, single-click the list. The list's properties will appear.
  4. Click Members. The members will be listed.