Many e-mail users like to write on electronic stationery to fancy up their messages.

True e-mail stationery fanatics are likely to have hundreds of stationery templates on their hard drives.
Eml, a relatively new format, allows them to pack lots of similar stationery templates - say, their holiday collection - into one easily managed file.

To read such an attachment, this may work:
Right-click on the attachment.
From the menu choose Save As
Save it as a separate file in a folder of your choice.
Open Internet Explorer
In the File menu, choose Open and select the attachment you saved as a file.