Fix a mailto Link That Opens Nothing or the Wrong App

A browser can block the handoff even when the handler is fine. Starting the URI directly separates the browser from the protocol registration.

Start-Process 'mailto:test@example.com?subject=Test'
Get-Process | Where-Object MainWindowTitle -match 'Mail|Outlook|Thunderbird' | Select-Object Name,MainWindowTitle
Get-WinEvent -LogName Application -MaxEvents 30 | Where-Object Message -match 'mailto' | Select-Object TimeCreated,Id,Message -First 3

The handler is a command with a placeholder. **%1** receives the address; without it the app starts but the fields stay empty, which looks like a broken link.

Get-ItemProperty 'HKCR:\mailto\shell\open\command' -ErrorAction SilentlyContinue | Select-Object '(default)'
Get-ItemProperty 'HKCR:\mailto' -ErrorAction SilentlyContinue | Select-Object '(default)','URL Protocol'
Get-ItemProperty 'HKCR:\outlookmail\shell\open\command' -ErrorAction SilentlyContinue | Select-Object '(default)'

A per-user ProgId overrides the machine registration and is protected by a hash. A wrong or stale entry here is the usual reason a correct handler is never used.

Get-ItemProperty 'HKCU:\Software\Microsoft\Windows\Shell\Associations\UrlAssociations\mailto\UserChoice' -ErrorAction SilentlyContinue | Select-Object ProgId,Hash
Get-ChildItem 'HKCU:\Software\Microsoft\Windows\Shell\Associations\UrlAssociations' | Select-Object PSChildName
Test-Path 'HKCU:\Software\Microsoft\Windows\Shell\Associations\UrlAssociations\mailto\UserChoice'

Only the Settings UI can write a valid hash. Picking the mail app there rebuilds the user choice and replaces a stale ProgId in one step.

Start-Process ms-settings:defaultapps
# Settings > Apps > Default apps > the mail app > Set default
# then scroll to the protocol list and confirm mailto
Get-ItemProperty 'HKCU:\Software\Microsoft\Windows\Shell\Associations\UrlAssociations\mailto\UserChoice' | Select-Object ProgId,Hash

An app that is installed but not repaired has no valid ProgId, so it never appears in the list. Repairing the installation rewrites the registration and the placeholder.

Get-ItemProperty 'HKCR:\Applications\OUTLOOK.EXE\shell\open\command' -ErrorAction SilentlyContinue | Select-Object '(default)'
Test-Path 'HKCR:\outlookmail\shell\open\command'
# Settings > Apps > Installed apps > the mail app > Advanced options > Repair

The test is a click, not a command. A mail window with the address and subject filled in means the placeholder and the choice are both correct.

Start-Process 'mailto:test@example.com?subject=Final%20test&body=Hello'
Get-ItemProperty 'HKCR:\mailto\shell\open\command' | Select-Object '(default)'
Get-ItemProperty 'HKCU:\Software\Microsoft\Windows\Shell\Associations\UrlAssociations\mailto\UserChoice' | Select-Object ProgId