Why won't the plug-in install on my machine?

When the Outlook installer fails, it is typically because one of the pre-requisite components (Visual Studio 2010 Tools for Office Runtime) is not properly installed. To resolve this issue, download and run the VSTO installer directly from Microsoft (use the "Run as Administrator" option):

https://www.microsoft.com/en-us/download/details.aspx?id=48217

After the VSTO installer completes, re-run the SendSafely Outlook installer to see if the problem is resolved.