More information at DMARC Guide.
docker-mailserver, DMARC is pre-configured out of the box. The only thing you need to do in order to enable it, is to add new
TXT entry to your DNS.
In contrast with DKIM, the DMARC DNS entry does not require any keys, but merely setting the [configuration values][dmarc-howto-configtags]. You can either handcraft the entry by yourself or use one of available generators (like this one).
Typically something like this should be good to start with (don't forget to replace
@example.com to your actual domain):
_dmarc.example.com. IN TXT "v=DMARC1; p=none; rua=mailto:firstname.lastname@example.org; ruf=mailto:email@example.com; sp=none; ri=86400"
Or a bit more strict policies (mind
_dmarc IN TXT "v=DMARC1; p=quarantine; rua=mailto:firstname.lastname@example.org; ruf=mailto:email@example.com; fo=0; adkim=r; aspf=r; pct=100; rf=afrf; ri=86400; sp=quarantine"
DMARC status is not being displayed instantly in Gmail for instance. If you want to check it directly after DNS entries, you can use some services around the Internet such as from Global Cyber Alliance or RedSift. In other cases, email clients will show "DMARC: PASS" in ~1 day or so.