Outgoing emails marked as SPAM and Phishing emails by O365 servers

ALV_Work We got exactly the same problem yesterday, I opened a ticket but Microsoft seems to have no clue about what happened. 

We worked on it a while and this is what we could figure out about our case :

- It started around 7am CET and ended around 8pm CET

- It has nothing to do with SPF or whatever

- EOP was giving a SFV:SPM SCL:5 to outbound emails ONLY IF they were replies or forwards to external email addresses, so they were using the High Risk Delivery Pool and we were getting a BCC in our IT mailbox as our Outbound spam Policy specifies it. I can see in your header that it was the same for you : DIR:OUT;SFP:1501;SCL:5

- When an external person was answering one on these emails they were coming back with SFV:SPM and CAT:PHSH, so we got PLENTY of emails yesterday ending up in junk folder

- Every Outbound Spam BCC in our IT mailbox arrived twice, the second time with a huge delay (6+ hours), and message stayed in "Getting Status" for very long time in traces, maybe because they were going through the High Risk Pool.

We didn't make any changes in our SPF or policies, it just randomly happened and ended.

I'm still waiting for an explanation from Microsoft.