All of a sudden (like a couple of days ago) all of our customers who are on Office 365 are getting our mails in their junk email. This is not the case with Gmail or other random mail servers, nor between us.

We got a 10/10 score on mail-tester.com. Also, I checked and our domain, quantic-conseil.com, is not blacklisted from what we saw on mxtoolbox.

Our SPF/DKIM/DMARC seem to be all good. So from what I saw in the header of a mail that was marked as spam in one of our customers'mailbox there is this little hint, as if one of Zoho's servers was blacklisted :

X-Forefront-Antispam-Report:

 CIP:135.84.80.217;IPV:NLI;CTRY:US;EFV:NLI;SFV:SPM;SFS:(2980300002)(286005)(350002)(199004)(189003)(349013);DIR:INB;SFP:;SCL:9;SRVR:QB1PR01MB3810;H:sender-of-o52.zoho.com;FPR:;SPF:Pass;LANG:fr;PTR:sender-of-o52.zoho.com;A:1;MX:1;CAT:BULK;

And out email is being marked as BULK.