This can happen for a number of reasons. In my experience it’s more often the case that a program didn’t properly clean up its registry entries than it is a group policy or other such rule (though you should probably verify GP settings in your specific case). Here are a few quick things to try.
- Navigate to Set Default Programs via the Control Panel (or however you want to get there). Set your default web browser to Internet Explorer. You can change it to something else once links are working in Outlook again.
- Find the following registry key: HKEY_CURRENT_USER\Software\Classes\.html Make sure its data value is htmlfile. Do the same for the .htm and .shtml keys while you’re at it.
- Find the following registry key: HKEY_LOCAL_MACHINE\SOFTWARE\Classes\htmlfile\shell\open\command. Make sure its data value is “C:\Program Files\Internet Explorer\IEXPLORE.EXE\” -nohome for 32-bit systems or “C:\Program Files (x86)\Internet Explorer\IEXPLORE.EXE” -nohome for 64-bit systems.
Check out this excellent post for more Outlook broken link fixes.