@polyfractal
Thank you for your help. You completely understood my problem. I tried to follow your guides, but it seems that it doesn't apply any analyzer different from the standard one. I paste here the function I'm using to put the mapping:
In message_body field I added the "analyzer": "english" and I run again the script which deletes any existing index with the same name and indexes again the set of documents.
Where am I wrong?