How do I set up authentication (SPF, DKIM) at the MTA level?
SPF is configured entirely in DNS, not the MTA. Add a TXT record to your domain specifying which IPs can send mail. For example: v=spf1 ip4:192.0.2.1 include:_spf.google.com -all. The MTA just needs to send from an IP listed in your SPF record.
DKIM requires MTA configuration to sign outgoing messages. The typical setup using OpenDKIM with Postfix:
- Generate a key pair:
opendkim-genkey -s selector -d yourdomain.com - Publish the public key in DNS as a TXT record at
selector._domainkey.yourdomain.com - Configure OpenDKIM with your private key path, selector, and signing domain
- Connect Postfix to OpenDKIM via milter:
smtpd_milters = inet:localhost:8891
Some MTAs have built-in DKIM signing without needing external tools. PowerMTA and KumoMTA both handle DKIM natively, so you configure the signing key and selector directly in the MTA, skipping the milter layer entirely.
Test your setup using tools like mail-tester.com or dkimvalidator.com to verify signatures are applied correctly and DNS records are published.
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