vivien, le SPF n'est toujours pas correct :
Authentication-Results: mx.google.com;
       spf=softfail (google.com: domain of transitioning ne-pas-repondre@lafibre.info does not designate 91.121.26.225 as permitted sender) smtp.mail=ne-pas-repondre@lafibre.info
L'enregistrement DNS :
lafibre.info    text =
        "google-site-verification=RpN76an0GUOCTewGW8QrVhPaiOaw5ei5aGG5uPb-eN8"
lafibre.info    text =
        "v=spf1 a mx ~all"
Les MX :
lafibre.info    MX preference = 1, mail exchanger = mx1.ovh.net
lafibre.info    MX preference = 5, mail exchanger = mx2.ovh.net
lafibre.info    MX preference = 100, mail exchanger = mxb.ovh.net
correspondent à 
Nom :    mx1.ovh.net
Address:  213.186.33.29
Nom :    mx2.ovh.net
Address:  213.186.33.45
Nom :    mxb.ovh.net
Addresses:  213.186.38.144, 213.186.39.173, 178.32.228.222, 213.186.35.149
          213.186.35.158, 213.186.37.67, 213.186.37.103
et pas à 91.121.26.225