Evolution allows to save e-mails as files via the "save as.." menu item. It should also allow opening/viewing these files with its email viewer component and register itself as handler for ".eml".

Thunderbird allows viewing .eml files (although not via double-clicking on the file, but via a menu item), as do all Windows-based e-mail clients (with integration in the shell, e.g. you can double-click an .eml file and it can be viewed).

I understand that I can open an .eml file with a text editor, but some e-mail specific features I cannot use this way:

 * Save attachments (I have to use mime tools in xterm)
 * Read e-mail with international characters (quoted-printable encoded)

Evolution already has the necessary code and UI, it just needs to support opening .eml files.