When emailing a link from the IBM Connections Files plugins for Notes, File links created by dragging the file into an email will not authenticate automatically in a SPNEGO environment.
When dragging a file from the Notes Sidebar from the Connections Plugins into a mail message, the link that is created uses the API to connect to the document. Currently it is not supported to authenticate using SPNEGO authentication through the Connections APIs.
Connections 3.x and 4.x with the Connections plug-in in the Notes 8 and 9 version clients.
Resolving the problem
Share the file directly with the users within the Files plugin, or open the file in the Browser and copy the URL from the browser directly to include in an email.