I have developed a custom protocol handler (let's call it "foo://") for use on my system by registering an executable at HKEY_CLASSES_ROOT\foo\shell\open\command. So far, so good.

I would like to send and receive links that use the foo:// protocol in emails. However, when I try to open one of these links in Outlook, I get the "This location may be unsafe" message below. I could just click "Yes" every time, but I would rather stop the message from appearing in the first place.

I've found instructions for how to disable the warning altogether (e.g. here - Add a DWORD value "DisableHyperlinkWarning" = 1 to HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\...\Common), but that's not what I'm after. Rather, I'd like to disable the warning on a per-protocol basis, so that my "foo://" protocol is whitelisted, but other protocols still show this warning when appropriate.

Can I do that? I'm using Outlook 2013.

1 Answer 1


Apparently, you can. This KB article describes the procedure (for 2007/2010, but the same thing works in 2013) in its "How to enable or disable hyperlink warnings per protocol" section.

In brief, to trust the "foo://" protocol, add a registry subkey named

HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\<version>\Common\Security\Trusted Protocols\All Applications\foo:

with no values under that subkey. This disables the warning for all Office applications (not just Outlook), which works fine for my purposes.

  • This works with Outlook 2016. The "Common" subkey did not exist before.
    – dummy
    Nov 22, 2019 at 14:50

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