Office 365 Customers Emails being silently blocked with certain Media[]Fire URL's.

We have heard from 3 different Office 365 customers that sending to or from Office 365 accounts using protection.outlook.com results in emails with certain media[]fire.com URL's being blocked or dropped. A similar incident happened several years ago.

Folder ULR's are not blocked, but image viewing URL's are:

hXXps://www[.]media[]fire[.]com/view/pxxxxxot3rflsw2/A44_Final_Logo%25282%2529.jpg/file

(URL changed for customer privacy).

We have tested through outlook.com to Gmail and from Gmail to Outlook without any issue so it seems specific to Office 365 customers since all impacted are using custom domains. i.e.

xxxx-com[.]mail.protection.outlook[.]com.

There doesn't seem to be any mechanism to report a false positive from outside of being an Office 365 admin and it seems the customers impacted are not familiar enough with these systems to investigate and make the report. How or who can we contact to have this false positive corrected?