We are using self-hosted Odoo. I checked on mail-tester and the problem seems to be FONT_INVIS_MSGID that means:

Invisible text + suspicious message ID  

I  suppose this is invisible text that odoo sends inside the message to help to classify the mail into the client conversations. if a send the email without using a CRM module thread, quote or invoice i get 10 points because there isn't this invisible code. So i'm pretty sure the code is the problem.

How can i fix this?

Thanks