Once you update the one on the Exchange server, you can test with OWA first to make sure client OAB updates will fix the issue.
If you are using Outlook 2003 or 2007 you may be in "Cached Mode", and you are looking at a copy called the "Offline address book" This only gets updated Every 24 hours, and the copy on the server only gets updated every 24 hours at (by default).
In this blog, I will share the difference, loop hole, and remediation.
A global address list allows both administrators and users to find fellow employees' email addresses and send them messages.
If using Outlook, query the GAL directly, or download the latest OAB. Before you do anything send the user you created a 'Test Email' from your own account.
The easiest way to do this is simply connect to Outlook Web Access, and query the GAL directly (OWA uses the GAL no the OAB like Outlook running in cached mode does). The User/Mailbox IS listed on the Server But NOT in Outlook.