After performing a migration from Lotus Notes to Exchange Online , Some users may see the following security pop ups when trying to click on doc links.
To resolve this issue the following items need to be added to a client machine’s registry.
1. Click Start, click Run, type regedit, and then click OK.
2. Locate the following registry subkey:
HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\xx.0\Common\Security\Trusted Protocols\All Applications
Note In this subkey, replace "xx.0" with 15.0 for Outlook 2013
3. Click the All Applications subkey.
4. On the Edit menu, point to New, and then click Key.
5. Type the name of the protocol that you want to exclude. For example, to disable the display of a security warning for the "Notes:" protocol, type Notes:.
Note Make sure that you include the colon (:) character.
Then use Group Policy Preferences to push it out to machines.
Ref: Article ID 925757