Ask HN: Why would Outlook clients suddenly quarantine loads of outgoing emails?

We’ve found that loads of emails we’re sending out to regular customers are getting quarantined by “anti-phishing” when sent from Outlook Desktop.

Does the fact we see these messages flagged and releasable in OUR security console mean it’s our system doing the quarantining? Why all of a sudden in the last few weeks? Has something changed in Outlook?

We’ve tried cycling our office IP address. No luck. Refreshing DKIM settings, nope. Enabling multi-factor auth, nada. Security score is in the high 80s.

Have you ever encountered quarantining of outbound Outlook mail like this?