Under Mathematica 11, I have cobbled together an Import Converter that is able to handle simple cases of Apple Mail emlx files. I did this because, without the Importer, emlx files are imported either as text or as HTML. Further, attempting to import by passing "EML" as the format, fails completely.
After placing Import.m and Converter.m in ~/Library/Mathematica/SystemFiles/Formats/EMLX,
$InputFormats does include 'emlx'.
EDIT: I have since determined that the minimum for a format to be listed in the results of $ImportFormats is a zero length Import.m file located within a directory matching the file extension within the SystemFiles/Formats directory. Which calls into question, IMO, the accuracy of the documentation covering
Unfortunately, emlx files still import as either HTML or Text. Ideally, I would want
FileFormat["foo.emlx"] to return
"EMLX" as its result. Currently, it returns HTML, TSV, CSV, or Text.
I have done the simple registration mechanism, also I tried to emulate that of the EML, and other example formats, yet still the Import framework is failing to automatically load the correct Converter. This leads me to believe that there is more that I have somehow missed to the registration process than placing
Converter.m, into a Format Directory and calling the registration mechanism.
The simplest registration I have done is :
ImportExport`RegisterImport[ "EMLX", EMLX`EMLXImportExport ];
Again, it works when explicitly referenced. Yet it doesn't seem to autoload for some reason.
Interestingly, Wolfram Desktop does not list the EMLX format at all. Because of this, I'm inclined to believe that Mathematica and Wolfram Desktop files belong under ~/Library/Application Support rather than at the top level of any domain Library folder. Simply put, that would make it clear that these are specific to the installed App rather than applying to Wolfram products as a whole.