Addressing DMARC issues with Mailchimp, Woodpecker, and O365 involves a comprehensive approach. It begins with configuring custom DKIM for Mailchimp and ensuring the SPF record correctly includes Mailchimp's domain and the O365 include if sending via Woodpecker. A DMARC record must be created in your domain's DNS settings. Monitoring sending and domain reputation is vital. Implementing DMARC requires correct SPF and DKIM configuration, and choosing an appropriate DMARC policy. Testing email authentication and regularly monitoring DMARC reports are crucial. Addressing common errors, verifying DMARC alignment, checking for organizational overrides, using SPF flattening, and considering testing on subdomains are key considerations.