That's interesting! Can you share which provider their MX record pointed to? Silently disappearing emails are typical Microsoft Forefront antics (but definitely

That's interesting! Can you share which provider their MX record pointed to? Silently disappearing emails are typical Microsoft Forefront antics (but definitely also a misfeature of other products), but mostly when the originating domain has a p=reject DMARC policy. You didn't happen to relax that at the same time you added the reporting parameters?