Here are some headers from message #33183 in this thread. (These are NOT from the July 29 message that Valimail flagged--I don't have those headers, unfortunately.) Looks like this one would have passed DMARC (both SPF and DKIM), which leaves me even more puzzled about the failure on July 29:
Authentication-Results: spf=pass (sender IP is 66.175.222.12)
?smtp.mailfrom=groups.io; mydomain.com; dkim=pass (signature was verified)
?header.d=groups.io;mydomain.com; dmarc=bestguesspass action=none
?header.from=groups.io;compauth=pass reason=109
Received-SPF: Pass (protection.outlook.com: domain of groups.io designates
?66.175.222.12 as permitted sender) receiver=protection.outlook.com;
?client-ip=66.175.222.12; helo=web01.groups.io;
From: "Mark Berry via groups.io" <myuser@...>
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=groups.io;
?q=dns/txt; s=20140610; t=1596580172;
?bh=quMW+/DLKq5/btYnRH7JLc0nNWFrv56txjhRwvbsWN0=;
?h=Content-Type:Date:From:Reply-To:Subject:To;
?b=DntI2eqhvN4xxekqtKPm+h8zB5A7tPoc7BSpNtwxs6SZVASg9BdKBm4D3InxVeE8VCR
?Y9OP9Zu64+VTGKx51iCAQIkaCcrjMTpMWNwvak2EbMOvBkDG3ihd7Jy+2nGJWqCet1gd4
?jQSNGxyiHe0apAKf78Fn9VmSTH2/q0aasN4=
