You need to publish a TXT record on your DNS to enable DMARC authentication.
1. Go to your domain provider (GoDaddy, Cloudflare, Namecheap, Gandi, etc.) and sign in.
2. Go to the page for updating your domain’s DNS records. It could be called "DNS Management", "Name Server Management", or "Advanced Settings".
3. Add this TXT Record to your DNS:
1. Name/Host/Alias: _dmarc
2. Time to Live (TTL): Enter 3600 or leave the default
3. Value/Answer/Destination: Replace {email} with your email address.
v=DMARC1; p=none; rua=mailto:{email}; pct=100; sp=none
You can also generate a DMARC record using this tool: DMARC Record Generator