Microsoft 365 groups external guests -DMARC fail

[rupie100]

We have external members in a M365 group.

1. ExternalMember1 sends an email to the group. ExternalMember1's domain has strict DMARC policies in place.

2. M365 delivers the message to ExternalMember2, who uses iCloud, with a new Sender Address auto-created by M365 (a complicated address combining the user and the group, with our domain).

3. DMARC reports the message to be blocked, and iCloud blocks the message because the sender is "spoofed".

At least that is what I think is happening (not my area of expertise). As of 06/2021, is there a way around this? Microsoft 365 groups and external guests are being used extensively, while at the same time strict DMARC policies are recommended. Still, having spent a couple of hours researching this subject, I currently think there's nothing I can do. Please prove me wrong.

[#2 · Accepted solution]

Answering to myself as I've been investigating this matter with Microsoft.

Update 9 June 2021:
Strong DMARC policy cannot be bypassed, i.e., in our scenario a Microsoft 365 group cannot be used to successfully distribute email to all recipients. The suggested workaround is to add the problematic recipients as CC (manually).

Microsoft has no ETA of resolving this limitation (I don't know if it's even possible) and as stronger DMARC rules are being recommended and honored across the web, it seems that using M365 groups + email + external guests is getting tricky.