Do you have a write-up of this, anywhere? I'd appreciate the details (what format did it reject? what did it change it to? what version of Exchange?).
Not OP but I've also seen this. I think some of the servers in question were "outlook-protection" in their domain names. Some kind of managed service middleware in the stack to do additional scanning.
I second the request. A few years ago we switched to Google Workplace, but it would be nice to know. I would like to forward it to the sysadmins just in case we go back to our own server.