microsoft 365 shared mailboxes not showing up in Outlook

If Microsoft 365 shared mailboxes aren't showing in Outlook, it's usually due to permissions, delayed syncing, or auto-mapping issues; first check with your admin for permissions and wait, then try restarting Outlook or manually adding the mailbox via File > Account Settings > Account Settings > More Settings > Advanced > Add, or use the "Add shared folder" option in the new Outlook, ensuring auto-mapping is enabled if needed via PowerShell. 

For Desktop Outlook (Classic/New)

1. Manual Add (Classic): Go to File > Account Settings > Account Settings, select your account, click Change, then More Settings > Advanced, and use Add to type the shared mailbox name.
2. Manual Add (New Outlook): Right-click your main account in the folder list (or click the three dots), select Add shared folder or mailbox, and type the name.
3. Check AutoMapping: If it still doesn't appear, ask your admin to run Add-MailboxPermission -Identity "SharedMailbox@domain.com" -User "User@domain.com" -AccessRights FullAccess -AutoMapping $true in Exchange Online PowerShell.