When a new email address or Distribution list is created, the email address or distribution list may not be visible in Outlook.

Its because Outlook may be running in Cached mode.

• Tools  > Options > Mail Setup
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• Click on Email Accounts
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• Double click your Email Account
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• UnCheck Use Cached Exchange Mode box
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• Click Next
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• Click Close
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• Open New Email, try the new email address to verify its seen.

Follow the same steps and Check the box to Use the Cached Exchange Mode.

Read more on benefits of using Cached Exchange Mode