Information
The Pickup directory is used by the Transport service on Mailbox servers and Edge Transport servers to insert message files directly into the transport pipeline. When properly formatted email message files are copied to the Pickup directory, they are submitted for delivery automatically. It is a legacy feature that can be used by administrators for mail flow testing, or by applications that must create and submit their own messages.
Ensure PickupDirectoryPath is set to $null
Rationale:
Disabling the Pickup directory is recommended to prevent potential abuse by attackers or insiders seeking to exploit side channel attacks. Dropping a file in the Pickup folder bypasses the normal authentication process of Exchange, creating a risk of sensitive data leakage or malicious email distribution through this unauthenticated side channel.
Impact:
The directory will be unavailable for testing purposes unless an administrator explicitly enables it and then later disables it. If an application requires access to this directory, then additional controls should be enabled such as restricting NTFS permissions on the folder.
Solution
To implement the recommended state, execute the following PowerShell cmdlet to disable the Pickup Directory on all servers with the Mailbox role
Get-TransportService | Set-TransportService -PickupDirectoryPath $null
NOTE: Edge Transport Servers must be configured directly.
Default Value:
%programfiles%\Microsoft\Exchange Server\V15\TransportRoles\Pickup