Our EmailFileToRasterFormat sample on github illustrates how to load an Email file (MSG or EML) and save it as a raster image. By default Rasterize() returns a 24 bit per pixel image. So, in order to save it to a format with a lower bit depth, you must specify a lower bit depth using the save options.
Note: ImageGear.Formats.Email functionality requires wkhtmltopdf. See Installing wkhtmltopdf for more information.
For conversion of Email to Multi-Page Image, one can convert an Email to PDF, then convert the resulting PDF to Multi-Page Image.