SPF, DKIM and DMARC: the what, the why and the how

Why you’d want to implement SPF, DKIM and/or DMARC

  • genuine emails from your domain are being marked as spam
  • you’ve become aware of (or want to prevent) emails being sent out, claiming to be from your domain

Background

example.com.  IN  MX  10  aspmx.l.google.com.

The problem of spam

  • it’ll give me a bad reputation with my users if they see dodgy emails appearing to come from me
  • spam filters from email providers will start to mark emails from example.com — including legitimate ones — as spam
  • when spam senders send out emails, including to invalid addresses, I’ll get ‘bounce back’ emails to my inbox — because the bounce back emails will go to the server listed in my MX record. This is known as ‘backscatter spam’ and will quickly get very annoying.

The problem of identifying senders

SPF — Sender Policy Framework

Implementing SPF

example.com.  IN  TXT  "v=spf1 include:amazonses.com ~all"

DKIM — DomainKeys Identified Mail

Implementing DKIM

w4brt6yt8k4lb7lvevow._domainkey.example.com  IN  CNAME w4brt6yt8k4lb7lvevow.dkim.amazonses.com

DMARC — Domain-based Message Authentication, Reporting and Conformance

  1. Check DKIM. If DKIM passes, DMARC passes. If DKIM fails or is not present, continue.
  2. Check SPF. If SPF fails, DMARC fails. If SPF passes, continue.
  3. Does the domain of the ‘envelope sender’/’Return-Path’ (the return address in our analogy) matches the domain of the ‘From’ address (the ‘From’ email is what appears in your email inbox)? If it matches, DMARC passes. Otherwise, DMARC fails.

Implementing DMARC

_dmarc.example.com  IN  TXT  "v=DMARC1;p=none;pct=100;rua=mailto:dmarcreports@example.com;"

Conclusion

Additional resources

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Keelan Fadden-Hopper

Keelan Fadden-Hopper

https://twitter.com/keelan_fh

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