In R5, under File -> Preferences -> Location Preferences, in the Internet Browser tab, you may choose either Notes as the Web browser, or an external browser.
When choosing Notes, you have the option of specifying whether retrieval is from the Notes workstation or via an interNotes server. This setting is hidden for external browsers.
The problem is, that when your Notes client is showing an HTML-based e-mail, it sometimes needs to fetch something from the Web (usually images), and it does so using the internal Notes browser using this setting no matter what your choice of browser is.
If, by chance, you use an external browser, and Notes is set-up to retrieve content via an interNotes server that doesn't exist or isn't configures, Notes will not be able to retrieve content, but you have no way of knowing about this setting because it's hidden.
This failure to retrieve contents usually results in missing images, but may sometimes cause Notes to fail to open the document (with an error saying that a file was not found on the server).
To modify the content retrieval setting switch the Internet Browser to Notes, change the setting, and then change the browser back to your choice.