Hi @Otani, Sora/大谷 蒼空,
Please run the following cmdlet in Exchange Management Shell on Exchange server to have a check if the system mailbox responsible for OAB generation is present or missing in the environment:
Get-Mailbox -Arbitration
Example:
This mailbox SystemMailbox{bb558c35-97f1-4cb9-8ff7-d53741dc928c} is responsible for generating OABs.
If this mailbox is missing, please follow the steps in this link to recreate it:
Re-create the Microsoft Exchange organization mailbox for OABs
If you can see this mailbox, please have a check of this path on Exchange server to see if you can see a folder named with guid:
%ExchangeInstallPath%ClientAccess\OAB (C:\Program Files\Microsoft\Exchange Server\V15\ClientAccess\OAB)
Example:
If there isn't such folder or the Date modified time is long time ago, please run the following cmdlet in Exchange Management Shell:
Get-OfflineAddressBook | Update-OfflineAddressBook
Then have a check if the folder is created or updated.
Meanwhile, check the application log in event viewer on the Exchange server for Event ID 17001-17004.
If you can see the folder and Event 17001 and 17002 on Exchange, please retry downloading the folder in Outlook.
If the issue persists, please check the url of the OAB virtual directory on Exchange server.
Example (in Exchange Admin Center>servers>virtual directories):
Make sure these urls are in the format of https://<server name>/OAB and accessible by Outlook client.
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