I did contact postmasters of free.fr with the errors you reported as well as a copy of the email I had receievd on my other email address. They replied to me that they need to receive the email directly from sender so that they can replicate the issue. IN parallel, I'm supposed to send the same email to a second address to let them know of the error.
The detailed process to be followed is:
resend the same email to my email address (****@free.fr) with nospam@nospam.proxad.net in BCC using the same tools than the "standard" email
then send another email to postmaster@proxad.net to let them know the issue detailing in it the sender, the topic and the error messages received.
If you're willing to follow that process, of course I'd be happy (and maybe it can be useful for other French people since free.fr is a common email provider). But if you don't, don't worry, I understand you're very busy and you have more exciting things to do.
I was thinking trying to do it myself by utsing the email address they mentioned, but I'd miss the BCC recipient.