S/MIME interoperability with external clients:: registry value externalsmimeUserHKCU\software\policies\microsoft\office\16.0\outlook\security

Policy ID: L_SMIMEinteroperabilitywithexternalclients
Scope: User Configuration
Supported on: At least Windows Server 2008 R2 or Windows 7
Source: Microsoft 365 Apps / Office Administrative Templates (version 5568.1000)
Category: Microsoft Outlook 2016 › Security › Cryptography

Registry

externalsmime

REG_DWORDUser

HKCU\software\policies\microsoft\office\16.0\outlook\security Current key

Enabled value: Depends on the option value
Element: Behavior for handling S/MIME messages:

Options
Item	Stored value
Handle internally	0
Handle externally	1
Handle if possible	2

Apply with .reg · CMD · PowerShell

.reg

Windows Registry Editor Version 5.00

[HKEY_CURRENT_USER\software\policies\microsoft\office\16.0\outlook\security]

; Options (use only one)
; Handle internally
"externalsmime"=dword:00000000
; Handle externally
"externalsmime"=dword:00000001
; Handle if possible
"externalsmime"=dword:00000002

CMD

:: Options (use only one)

:: Handle internally
reg add "HKCU\software\policies\microsoft\office\16.0\outlook\security" /v "externalsmime" /t REG_DWORD /d 0 /f
:: Handle externally
reg add "HKCU\software\policies\microsoft\office\16.0\outlook\security" /v "externalsmime" /t REG_DWORD /d 1 /f
:: Handle if possible
reg add "HKCU\software\policies\microsoft\office\16.0\outlook\security" /v "externalsmime" /t REG_DWORD /d 2 /f

PowerShell

# Common: create the key
New-Item -Path 'HKCU:\software\policies\microsoft\office\16.0\outlook\security' -Force | Out-Null

# Options (use only one)
# Handle internally
New-ItemProperty -Path 'HKCU:\software\policies\microsoft\office\16.0\outlook\security' -Name 'externalsmime' -PropertyType DWord -Value 0 -Force | Out-Null
# Handle externally
New-ItemProperty -Path 'HKCU:\software\policies\microsoft\office\16.0\outlook\security' -Name 'externalsmime' -PropertyType DWord -Value 1 -Force | Out-Null
# Handle if possible
New-ItemProperty -Path 'HKCU:\software\policies\microsoft\office\16.0\outlook\security' -Name 'externalsmime' -PropertyType DWord -Value 2 -Force | Out-Null