DMARC’s alignment feature prevents spoofing of the “header from” address by:
- Matching the “header from” domain name with the “envelope from” domain name used during an SPF check, and
- Matching the “header from” domain name with the “d= domain name” in the DKIM signature.
To pass DMARC, a message must pass SPF authentication and SPF alignment and/or DKIM authentication and DKIM alignment. A message will fail DMARC if the message fails both (1) SPF or SPF alignment and (2) DKIM or DKIM alignment.
DMARC allows senders to instruct email providers on how to handle unauthenticated mail via a DMARC policy, removing any guesswork on how they should handle messages that fail DMARC authentication. Senders can either:
- Monitor all mail, to understand their brand’s email authentication ecosystem, and ensure legitimate mail is authenticating properly without interfering with the delivery of messages that fail DMARC
- Quarantine messages that fail DMARC (e.g., move to the spam folder)
- Reject messages that fail DMARC (e.g., don’t deliver the mail at all)
Mailbox providers send regular DMARC aggregate and forensic reports back to senders, giving them visibility into what messages are authenticating, what messages are not, and why
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