Policy
Attachment Secure Temporary Folder
This policy setting allows you to specify a folder path for the Secure Temporary Files folder rather than using the one that is randomly generated by Outlook. If you enable this policy setting, you can specify a folder path for the Security Temporary Files folder rather than using the one that is randomly generated by Outlook. If you disable or do not configure this policy setting, Outlook will assign the Secure Temporary Files folder a different random name for each user. Important - If you must use a specific folder for Outlook attachments, Microsoft recommends that you use a local directory (for best performance), that you place the folder under the Temporary Internet Files folder (to benefit from the enhanced security on that folder), and that the folder name is unique and difficult to guess.
00467016e8a1 L_AttachmentSecureTemporaryFolder Registry
Copy registry mappings
HKCU\software\policies\microsoft\office\16.0\outlook\security\outlooksecuretempfolderPolicy notes
This policy setting allows you to specify a folder path for the Secure Temporary Files folder rather than using the one that is randomly generated by Outlook. If you enable this policy setting, you can specify a folder path for the Security Temporary Files folder rather than using the one that is randomly generated by Outlook. If you disable or do not configure this policy setting, Outlook will assign the Secure Temporary Files folder a different random name for each user. Important - If you must use a specific folder for Outlook attachments, Microsoft recommends that you use a local directory (for best performance), that you place the folder under the Temporary Internet Files folder (to benefit from the enhanced security on that folder), and that the folder name is unique and difficult to guess.