Forwarding email with outlook.com/Office365/Microsoft Exchange also breaks DMARC. Exchange Server sometimes modifies the headers of emails when forwarding them, invalidating the DKIM signature, so then a DMARC policy rejects the forwarded email. Apparently Microsoft have a fix in the pipeline for this, but it's been taking ages.
More info at: https://blogs.msdn.microsoft.com/tzink/2016/05/19/why-does-m...