Prevent users from customizing attachment security settings
This per-user policy value prevents users from overriding the set of attachment types blocked by Outlook through their own attachment security settings.
since 0.9.8.11
Purpose
Show registry keys (1)
HKCU\software\policies\microsoft\office\16.0\outlook\disallowattachmentcustomization
This per-user policy value prevents users from overriding the set of attachment types blocked by Outlook through their own attachment security settings. When the policy is enabled, it is represented by the disallowattachmentcustomization value of type REG_DWORD with data 1.
The setting does not define a list of file-name extensions itself. Outlook derives the effective block list from its built-in attachment security rules and any applicable administrative policies. Outlook also evaluates related settings such as Level1Remove when determining attachment behavior.
Notes
Security
The policy prevents users from independently weakening Outlook's restrictions on potentially dangerous attachment types. This preserves the intended protection against attachments that may contain executable code, scripts, or similar active content.
It is not a malware scanner and does not block every attachment. It also does not prevent access to equivalent files outside Outlook or protect against file types that are not covered by the effective attachment security rules.
Possible side effects
Legitimate files remain inaccessible through Outlook when their file type is on the effective block list and a user-specific exception would otherwise have been possible. Workflows that depend on such attachments can therefore be disrupted.
Notes
The value documented here resides directly under HKCU\software\policies\microsoft\office\16.0\outlook. It should not be confused with the legacy value of the same name under ...\outlook\security documented by Microsoft for earlier Outlook versions. The two registry locations are not interchangeable.
Microsoft documentation
Microsoft Download Center: Administrative Template files (ADMX/ADML) for Microsoft Office
Microsoft Support: Blocked attachments in Outlook
Microsoft Learn: Information for administrators about e-mail security settings in Outlook 2007