Hello, 

Currently webmail does not have the ability to open up emails that are not in the Mailbox of the user. There are a few options to open this message to read it. 

If you would like I can get this escalated as a feature request for an EML viewer option in webmail. 

Thank you

~Tony

I would very much like to add this to the feature requests for sure, my users get so confused and they hate the 3rd party client option (and I usually have to explain how to do it every time)

+1 for the feature request.  We've had to temporarily add accounts to Outlook to inject EML files.  Would have been nice to have been able to impersonate the user (or walk them through it) to pull EMLs into a folder in Webmail.