Exchange Unicode Mode - Ignore OST Format: registry value ignoreostformatUserHKCU\software\policies\microsoft\office\16.0\outlook\emsp

Policy ID: L_ExchangeUnicodeModeIgnoreOSTFormat
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 › Account Settings › Exchange

Registry

ignoreostformat

REG_DWORDUser

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

Enabled value: Depends on the option value
Element: Choose whether existing OST format determines mailbox mode

Options
Item	Stored value
OST Format determines mode	0
Create new OST if format doesn't match mode	1
Prompt to create new OST if format doesn't match mode	2

Apply with .reg · CMD · PowerShell

.reg

Windows Registry Editor Version 5.00

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

; Options (use only one)
; OST Format determines mode
"ignoreostformat"=dword:00000000
; Create new OST if format doesn't match mode
"ignoreostformat"=dword:00000001
; Prompt to create new OST if format doesn't match mode
"ignoreostformat"=dword:00000002

CMD

:: Options (use only one)

:: OST Format determines mode
reg add "HKCU\software\policies\microsoft\office\16.0\outlook\emsp" /v "ignoreostformat" /t REG_DWORD /d 0 /f
:: Create new OST if format doesn't match mode
reg add "HKCU\software\policies\microsoft\office\16.0\outlook\emsp" /v "ignoreostformat" /t REG_DWORD /d 1 /f
:: Prompt to create new OST if format doesn't match mode
reg add "HKCU\software\policies\microsoft\office\16.0\outlook\emsp" /v "ignoreostformat" /t REG_DWORD /d 2 /f

PowerShell

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

# Options (use only one)
# OST Format determines mode
New-ItemProperty -Path 'HKCU:\software\policies\microsoft\office\16.0\outlook\emsp' -Name 'ignoreostformat' -PropertyType DWord -Value 0 -Force | Out-Null
# Create new OST if format doesn't match mode
New-ItemProperty -Path 'HKCU:\software\policies\microsoft\office\16.0\outlook\emsp' -Name 'ignoreostformat' -PropertyType DWord -Value 1 -Force | Out-Null
# Prompt to create new OST if format doesn't match mode
New-ItemProperty -Path 'HKCU:\software\policies\microsoft\office\16.0\outlook\emsp' -Name 'ignoreostformat' -PropertyType DWord -Value 2 -Force | Out-Null