When Topicbox receives a message, we first check the DMARC policy of the sending domain. If we find a strict policy for that domain (p=quarantine or p=reject), we rewrite the From header to avoid delivery problems.
If you have a DMARC policy of p=none, you may notice DMARC failures from Topicbox in your DMARC summary reports. Nearly all mailing lists break DKIM signatures because they alter the message body. Topicbox also does this when we add a footer to messages you send to your group.
If your domain's SPF record does not include the IP addresses that Topicbox uses to send, this message rewriting combined with an SPF failure can cause a DMARC failure.