As a lawyer required to use e-filing, I receive numerous email notices from the court daily. In Outlook I save those messages to case folders in my file system using an Outlook plugin called "Message Save." The stored messages are in ".eml" format. I view the messages in the Directory Opus viewer pane to determine what action, if any, is needed. Often a message requires me to retrieve an document from the court website, with a link to the document being provided. Clicking a link in the viewer pane does not work, so I have to double-click the email message itself, which opens the message in Outlook, where I can click the link and access the online document. This gets tedious when I have multiple documents to download, so it would be much handier if there were a way to activate links in the Opus viewer pane.
The .eml preview handler is not part of Opus; it's part of either Windows or Outlook, and just something Opus can use if it's there.
You should see the same results in File Explorer's viewer pane, as it should use the same preview handler.
Sometimes there are other preview handlers you can try for the same format. In this case, go to Preferences / Viewer / Plugins, select the ActiveX + Preview + Office + Web plugin and click Configure. You'll see a list of possible preview handlers there.
Others that involve email or MAPI may be worth a try in case they work better. But none of the preview handlers listed there are written by us and we can't control how they behave. We only have control over the internal viewer and the viewer plugins which come with Opus.
(I think Microsoft locked down their preview handlers a lot in the name of security, which may be why it won't follow links when clicked.)