The design of Sharepoint 2010 contains several new security features. Opening certain files types in browser view is prohibited by default. One of which is the opening of .eml files.
To change this setting do the following:
- Go to 'Manage Web Applications' in the Central Administration Console -
- Click 'General Settings'
- set 'Browser File Handling' to 'Permissive'.
Attention: only adjust this setting if you are really confident the files are safe. Forcing files to download is a security measure to prevent the running of mallicious code.