Policies
You may need to be more restrictive to make your server more secure. There are policies to do this for you. Read particular options carefully as they offer you powerful ways to secure your users against attacks or misuse. If your policies are set incorrectly, it may take some time to find out the causes of your problems. (e.g.: if you setup your login policy with the email address option chosen, and then your users try to login with their common usernames, they will no longer be able to log on until you resolve your policy settings).
Figure. Policies management.