Mailman needs a setting to apply DMARC mitigations to specific From: addresses.
Mailman 2.1 has a setting to apply DMARC mitigations to messages From: addresses matching a listed address or regexp regardless of the From: domain's published DMARC policy. This is needed in Mailman 3 because Gmail has started rejecting messages From: gmail.com that fail DMARC in spite of it's published DMARC p=none. A typical rejection is
SMTP error from remote mail server after end of data:
550-5.7.26 Unauthenticated email from gmail.com is not accepted due to domain's
550-5.7.26 DMARC policy. Please contact the administrator of gmail.com domain
550-5.7.26 if this was a legitimate mail. Please visit
550-5.7.26 https://support.google.com/mail/answer/2451690 to learn about the
550 5.7.26 DMARC initiative.
This is a significant mail delivery issue.