Set Outlook as default MailTo application

If you are an administrator and are responsible for the deployment and maintenance of Microsoft Office in your organization, it is almost certain that at some time you have been asked to customize Microsoft Outlook.

This post focuses on one specific configuration option that you can set by default: configuring Windows clients to use Outlook as the default MailTo email program.

Although individual users can manually set this option in Windows Settings | Choose a default app for each protocol, this assumes that the user is familiar with the setting. Otherwise, this can result in calls to the help desk.

Before Windows 8, you could control the handling of the MailTo protocol by deploying a single registry value or group policy, thus setting outlook.exe to Default:

HKEY_CLASSES_ROOT\mailto\shell\open\command

However, the above method does not work in Windows 8 and Windows 10. The default application for the MailTo protocol is now stored here:

HKEY_CURRENT_USER\Software\Microsoft\Windows\Shell\Associations\URLAssociations\MAILTO\UserChoice

Since Windows uses a hash to protect this new registry location, you cannot simply edit the values stored in that path. Instead, you must use an XML-based group policy to define the set of default apps.

Create your App Associations XML

1. Manually configure Default Apps in Windows Settings on your source PC
   1. For MailTo, click Choose default apps by protocol then find MAILTO in the list. Set Outlook as the default app:

2. Once the defaults are set, use DISM to export these values to XML

From an administrator command prompt:

Dism /Online /Export-DefaultAppAssociations:"F:\AppAssociations.xml"

Note The XML will be based on the defaults set for the user account used when running DISM

Note For Outlook to be the default for opening MailTo links, you should see a line like this in the XML:

<Association Identifier="mailto" ProgId="Outlook.URL.mailto.15" ApplicationName="Outlook 2016" />

Deploy your App Associations XML via GPO

The name of the policy to use is Set a default associations configuration file and is found in the administration templates at this path:

Computer Configuration\Administrative Templates\Windows Components\File Explorer

1. Set this policy to Enabled
2. Specify the path to the app associations file that you previously created

Note This can be a local path or a network location that all clients can access

3. This policy should create the following key:

HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\System

String Value: DefaultAssociationsConfiguration

Data:\\ServerPath\AppAssociations.xml

Note This policy only works for domain joined clients

After this policy is in place, the default application associations are applied once the user logon occurs. The user should now see MailTo links open an Outlook email message.