Knowledge Base

How to fix problems with installation of CodeTwo Email Signatures for Office 365 desktop components


You are not able to install the Manage Signatures App and the following error appears:

Your administrator has blocked this application because it potentially poses a security risk to your computer.
Your security settings do not allow this application to be installed on your computer.

Fig. 1. The error window.

You experience this error even when you try to install the app as an administrator.


This problem is related to security options on your Windows machine, not to CodeTwo software. You can experience this issue in various scenarios:

  • when the group policy (GPO) in your organization is set to block ClickOnce applications;
  • if your organization uses third-party tools, these tools might block the installation of other applications;
  • if you have not experienced this issue before, the security settings on your machine might have changed after a recent Windows update.

Usually, the problem does not appear in clean Windows installations and might mean that the security settings on your machine are different from the default Windows configuration.

To solve this problem, you need to:

How to add the URL of the Manage Signatures App aps to Trusted Sites in Windows

To fix problems with the installation of the Manage Signatures App (which is a ClickOnce app), try updating the URL security zones in Windows: you need to specify that the CodeTwo app comes from a trusted source. You can do that by adding the URL for CodeTwo Email Signatures for Office 365 to Trusted Sites in the Internet security options:

  1. Open Windows Control Panel and go to Internet options (you can also access these settings by opening the Start menu, typing Internet options and pressing Enter).
  2. Click the Security tab and select the Trusted Sites zone.
  3. Click the Sites button on the right side and add the following URL: as shown in Fig. 2.

Fig. 2. Adding CodeTwo to Trusted Sites.

  1. Click OK to save the changes.

You should now be able to install the Manage Signatures App. If the installation window (the ClickOnce trust prompt) does not appear, see section Manage the ClickOnce trust prompt behavior on your machine.

Manage the ClickOnce trust prompt behavior on your machine

The ClickOnce trust prompt is displayed when you install or launch a ClickOnce app. It gives you the option to agree or disagree to install or run the app. Sometimes, the prompt may be disabled for certain URL security zones in Windows. Follow the steps below to check if the ClickOnce trust prompt is enabled for Trusted Sites, and, if necessary, modify these settings.


This solution requires modifications to Windows Registry. It may be a good idea to export a backup copy of the registry before you proceed.

  1. Open Windows Registry Editor (e.g. by clicking the Start button and typing regedit).
  2. Find the following key in the registry:
  3. Find the following entry (value): TrustedSites. If its value is set to Disabled, change it to Enabled. (If the registry key shown in step 2. does not exist, you need to create it first. Then you need to add the TrustedSites subkey as String Value.)


This change enables your machine to download and install ClickOnce applications from the URLs defined in the Trusted Sites security zone in Windows (see Fig. 2.). This configuration is the most reasonable choice to allow only specific apps to be downloaded and installed while maintaining high security standards within your company.

For more information on these settings, see the MS article on configuring ClickOnce Trust Prompt.

See also:

Manage Signatures App startup and login problems