Everyone that is active in the Active Directory (AD) (i.e., people with a left_uiuc field are not included), will show up in the Global Address List (GAL).

Information in the GAL is only updated once a day from AD. AD service managers are responsible for the GAL data, though Exchange service managers have access as well.

Microsoft Outlook clients use a cached version of the GAL, called an Offline Address Book that is synced with the GAL at a nearly daily interval. So, it can take multiple days before a change at the Enterprise level (Banner/My UI Info [formerly Nessie]) to propagate and be visible in Outlook. In general, Outlook on the Web offers realtime updates, while Outlook takes 26 hours. Outlook can be manually updated via the other solution: Office 365, Email, Exchange, Offline Global Address List needs to be synchronized

That is, you can send email to any user in AD. If they have an Exchange account, then they are "mailbox-enabled" and the GAL determines which Exchange Server hosts their mailbox. If they do not have an Exchange account (they are still mail-enabled), the GAL says their email is "netid@illinois.edu" and is told to route that mail back up to the relays, and then it's up to the relays to look up that email redirection field and deliver their mail appropriately.