Using classic Outlook for Windows in business environments
Based on my research, in classic Outlook the shared folder may take some time before it is automatically synchronized and becomes visible.
In the meantime, you may try adding it manually by following these steps:
- Go to File > Account Settings > Account Settings
- Select your account > Change > More Settings > Advanced
- Choose Add > enter the email address of the shared mailbox > OK
Once completed, the shared folder should appear in your folder list.
In addition, I would kindly suggest confirming that the mailbox owner has granted the appropriate permissions. The shared folder needs to be enabled both at the mailbox level and within the folder permissions, as described in the following guide: Share and access another person's mailbox or folder in Outlook | Microsoft Support
I hope this brings some helpful clarification.
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