Signature Warning
This registry value controls how Outlook warns users when they open an email message that has an invalid digital signature.
since 0.9.8.11
Purpose
Show registry keys (1)
HKCU\software\policies\microsoft\office\16.0\outlook\security\warnaboutinvalid
This registry value controls how Outlook warns users when they open an email message that has an invalid digital signature. It is a per-user policy value of type REG_DWORD.
The supported data values have these meanings:
0: The user can decide whether Outlook should continue to display warnings for invalid signatures.1: Outlook always warns about invalid signatures.2: Outlook does not display warnings for invalid signatures.
The setting controls only the warning behavior. It does not change cryptographic signature validation, make an invalid signature valid, or establish trust in the affected message.
Notes
Security
A value of 1 preserves the warning even when users could otherwise choose not to see future warnings. Invalid signatures therefore remain visibly identified.
With 0, users can suppress subsequent warnings. With 2, Outlook suppresses the warning entirely. Either behavior increases the possibility that an invalid or potentially fraudulent signature will go unnoticed and that the message will be mistaken for an authentic one.
Possible side effects
With a value of 1, Outlook displays the warning dialog for every affected message. This can repeatedly interrupt users who receive many messages with invalid signatures.
Notes
The 16.0 policy branch is shared by the administrative templates for Outlook 2016 and later classic Outlook releases in the same Office policy family.
Microsoft documentation
Microsoft Learn: Microsoft 365 Apps security baseline settings – Signature Warning
Microsoft Download Center: Administrative Template files for Microsoft Office