In `docker-mailserver` DMARC is configured out-of the box and the only thing you need to do is to add new TXT entry to your DNS. In contrast with [DKIM](https://github.com/tomav/docker-mailserver/wiki/Configure-DKIM), DMARC DNS entry does not require any keys but just setting the [configuration values](https://github.com/internetstandards/toolbox-wiki/blob/master/DMARC-how-to.md#overview-of-dmarc-configuration-tags). You can either handcraft the entry by yourself or use one of available generators (like https://dmarcguide.globalcyberalliance.org/).
Typically something like this should be good to start with (don't forget to replace `domain.com` to valid addresses)
DMARC status is not being displayed instantly in Gmail. If you want to check it directly after DNS entries, you can use some services around the Internet such as mentioned https://dmarcguide.globalcyberalliance.org/ or https://ondmarc.redsift.com/. In other case Gmail will show "DMARC: PASS" in ~1 day or so.
