Global Address List in Exchange Server 2003

It's natural that users try and locate each others email addresses in the GAL. But why would an administrator need to configure Exchange 2003's GAL? The most likely answers are:
1) To check permissions.
2) Create custom lists.
3) Control how the names are displayed in the Global Address List.
Just go with the default GAL (Global Address List). Exchange 2003 server automatically adds every new mail recipient to the Global Address List
Objects found in the Exchange 2003 GAL

Pay careful attention to the difference between a mailbox enabled user and a mail-enabled user; a security group and a distribution group. Mailbox enabled accounts. Regular users with MAPI mailboxes Active Directory accounts.

Mail-enabled users. Contractors who have an Active Directory logon but no mailbox. (No mailbox in your Exchange Organization.) Contacts. Suppliers, customers, people with email address outside your organization. No Active Directory account.

Distribution groups. These can be Global or Universal Groups, but they are designed for email rather than security. These are sometimes referred to as DLs - Distribution lists instead of distribution groups.

Security groups that have mailboxes. People will say that unless you have a good reason, favour the classic Distribution group and avoid Mail Enabled Security Groups.

Public Folders. Mail-enabled public folders if your users need an easy way to post.