Maybe check your options in the client.

These instructions are based on the Windows Client - I don't use the Web Client, and I'm not certain what will be different in that environment.

Go to Tools > Options.  Then in the Options dialog, go to View > Open With.

In my case, if I have it set to "Open electronic documents by default using: The Laserfiche Document Viewer" then it will try to open in the Laserfiche viewer, but I don't see anything unless I have generated pages.  But if instead, I have it set to "Open electronic documents by default using: The associated application" then it opens right up in Outlook without an issue.

Can you check that setting in your environment?

Also, note that what you said about losing the attachments when you generate pages - it is not losing the attachments, it is just adding the Laserfiche pages in addition to the email and attachments and those pages will want to open in the Laserfiche Viewer instead of the default application.  But the electronic document component is still there in addition to the images. Regardless of the setting I mentioned before, and in the case of generated pages - you can always open the electronic document by right-clicking and selecting Open > Edit Electronic File or Open > View Electronic File.  

EDIT TO ADD:  The options also can address how the email attachments are handled.  under New Documents > Outlook, you can change the default behavior for attachments to chose to have them as separate Laserfiche documents.  At the default of "Leave in e-mail document" I do not believe you would ever see the attachments when you generate pages.

It seems like you're not the only one who ran into Outlook refusing to open msg files: https://answers.microsoft.com/en-us/outlook_com/forum/all/outlook-2016-wont-open-saved-msg-or-pst-files/77cc33eb-6f39-49e8-a1f2-c447e042b9b9 is one of the first hits, but there are several posts on Microsoft's forums.