As many users have discovered, the Outlook:// protocol no longer works outside of Outlook 2007 (and up). This protocol allows users to create shortcuts to Outlook items and open the items outside of Outlook. You can use this protocol on a folder home page, on the Web toolbar, or in the body of an Outlook item but not in a web page that does not open in a folder homepage.

See Using Outlook Links to learn how to use the Outlook:// protocol

This is because the Office installer no longer registers Outlook to a URL protocol. This is due to Microsoft not officially supporting the Outlook URL protocol as explained in KB 929590 Fortunately, Outlook still supports it, but to use it, you will need to add it as a URL protocol in the Windows Registry.

The Fix

Note: It is recommended that your make a backup copy of your registry before editing the registry.

The steps for Outlook 2010, 2013, and 2016 are the same, except you will the correct path to Outlook.exe. In Outlook 2016, it's C:\Program Files (x86)\Microsoft Office\root\Office16\outlook.exe, if using Click-to-run (most people will have click to run).

1. Open the registry editor and select root key HKEY_CLASS_ROOT
2. Add a new Key named “outlook”
3. Edit the (Default) string value and add URL:Outlook Folders
4. Add the new string value “URL Protocol”, no string value is required
5. Add subkeys \shell\open\command
6. Select the Default Icon key and enter:
7. C:\Program Files\Microsoft Office\Office12\OUTLOOK.EXE
   Note: If you are using a 64-bit Windows operating system with 32-bit Outlook, then instead of “Program Files” use “Program Files (x86)”. Don't forget to use the correct version of Office!
8. Select the command key value under shell\open, edit the (Default) string value and add the following string: (Note this assumes you installed Office in the default location/) You need to include the double quotes in the string.
   "C:\Program Files\Microsoft Office\Office12\OUTLOOK.EXE" /select "%1"

You can test it by clicking here. If Outlook opens to your Inbox, the keys were created correctly.

Do It For Me

To use, download the correct file for your system then double click to add the key to your registry.

If you are using 32-bit Outlook on 64-bit Windows:
Outlook 2016/2019/365 (32-bit/Wx64)Outlook 2013 (32-bit/Wx64)
Outlook 2010 (32-bit/Wx64)Outlook 2007 (32-bit/Wx64)

If you are using the same "bitness" of Windows and Outlook (both are 32-bit or both are 64-bit):
Outlook 2016/2019/365Outlook 2013
Outlook 2010Outlook 2007

Note: Outlook does not automatically recognize the Outlook protocol when you type a url containing it into the body of any item. However, the protocol works if you set it as a hyperlink.