Display Level 1 attachments
This per-user `REG_DWORD` value controls whether attachments whose file types are classified as Level 1 are accessible in classic Outlook.
since 0.9.8.11
Purpose
Show registry keys (1)
HKCU\software\policies\microsoft\office\16.0\outlook\security\showlevel1attach
This per-user REG_DWORD value controls whether attachments whose file types are classified as Level 1 are accessible in classic Outlook. Outlook assigns potentially dangerous file types to Level 1 and normally blocks access to them completely.
When showlevel1attach = 1, Outlook displays Level 1 attachments. Users can save them to disk and then open them from that location, so they are handled like Level 2 attachments for access purposes. They still cannot be opened directly from Outlook.
When showlevel1attach = 0, Level 1 attachments are not displayed. Outlook also blocks access to these attachments by default when the value is not configured.
The value does not determine which file name extensions belong to Level 1. That classification comes from Outlook's built-in lists and any changes made through separate attachment-security policies.
Notes
Security
A value of 1 reduces Outlook's protection against potentially dangerous email attachments. Executables, scripts, and other file types classified as Level 1 can consequently be saved and opened outside Outlook, increasing the risk that a user will run malicious content received in a message.
The setting does not execute attachments automatically or bypass operating-system and endpoint-security controls. Opening or running a saved file still requires user action, and file permissions, Microsoft Defender, SmartScreen, and other applicable protections remain in effect.
Notes
The Office\16.0 policy path applies to classic Outlook. Microsoft states that Outlook ADMX policies do not apply to new Outlook for Windows, where attachment handling is governed by different policies.
As a dependent Outlook security policy, this setting is evaluated only when Outlook is configured to use Outlook security settings supplied through Group Policy.
Microsoft documentation
Microsoft Learn: Information for administrators about email security settings in Outlook
Microsoft Learn: Map classic Outlook policies to new Outlook
Microsoft Support: Outlook known issues in the June 2017 security updates