Quoting info at smallinnovations dot nl (info@???):

> So far i have just installed DMARC one time but if i remember it
> correctly either SPF or DKIM had to be correct to accept the e-mail or
> not. To quote my source "A message will fail DMARC if the message fails
> both (1) SPF or SPF alignment and (2) DKIM or DKIM alignment.". DKIM
> would be a hard nut to crack for MLM but SPF should not be a problem
> then? Or do I overlook something?

Why messages fail DMARC is convoluted, and I'd frankly rather spend my
time on other things. If you are wanting to spend a lot more time on
this, here's a fine place to start:
https://wiki.asrg.sp.am/wiki/Mitigating_DMARC_damage_to_third_party_mail