DMARC p=reject Without Breaking Email
Email authentication remains one of the most critical challenges for organizations seeking to protect their domains from spoofing and phishing attacks. This article explores practical strategies for implementing DMARC p=reject policies without disrupting legitimate email flows, drawing on proven methods from email security specialists. Learn how to enforce strict matching to block typosquatting attempts and gradually phase in policies to manage subdomains effectively.
Enforce Strict Match Block Typosquats
Honestly, moving to DMARC p=reject wasn't that scary. We just watched the reports for a couple weeks first. The big thing I noticed was switching from relaxed to strict DKIM alignment caught way more spoofing, especially from typo subdomains. Using DMARCian to track it, our spam alerts went from a weekly thing to basically nothing. Start with p=none and study the reports closely before you make the jump.
Phase Policy Slowly Tame Subdomains
In my cybersecurity role, we learned not to rush DMARC. We slowly moved from p=none to p=reject, checking reports each week and fixing SPF and DKIM records. This avoided any major email outages. The sp setting also caught some spoofing from subdomains we would have missed. The biggest change was easyDMARC. Our spam complaints went from about 7 a week to, well, almost zero. That happened within two months.
Enable ARC Across Hops
Authenticated Received Chain helps receivers trust mail that passes through lists and gateways that change the body. Turn on ARC sealing on any relay or list you operate so downstream sites see the original pass results. Ask key partners and vendors to enable ARC so their hops do not undo your work. Validate ARC on your inbound edge to inform routing choices and lower false rejects.
Keep DKIM and SPF in place because ARC adds context but does not replace them. Review DMARC failure samples to confirm ARC is present when content gets changed in transit. Enable ARC across your path and run staged tests before enforcing reject.
Split Marketing And Transactional Domains
Using different domains for marketing and transactional mail lowers risk and makes policy simple. Put receipts, alerts, and codes on the primary domain with strict alignment and a strong DMARC policy. Send bulk promos from a branded subdomain that can ramp from none to quarantine to reject on its own timeline. This split prevents a mistake in one stream from hurting delivery for the other stream.
It also lets each stream have its own keys, IPs, and feedback loops without cross impact. Reporting stays clear because each domain shows only its own traffic. Carve out the streams today and publish separate DMARC, SPF, and DKIM for each.
Map Senders Then Lock Access
Start with a full map of every system that sends mail for the domain, including SaaS tools, support desks, and devices. For each sender, enable DKIM signing with a d= domain that matches the visible From domain. Add only needed includes to SPF and remove old or unknown entries to cut spoofing risk. Lock unauthorized sending by refusing API keys and SMTP creds for unknown apps and by narrowing allowed from addresses.
Use DMARC aggregate reports to find stray sources and fix or shut them down. Move policy from none to quarantine and then to reject once all sources pass. Begin by listing every mail sender today and scheduling owners to fix alignment.
Favor DKIM For Forwarded Mail
Forwarding often changes the sending IP, which breaks SPF, while DKIM survives those hops. Make each sender sign with DKIM using a key that aligns with your From domain. Keep keys strong and rotate them on a regular schedule to limit risk from leaks. Ensure vendors use one aligned domain rather than their default shared domain to avoid misalignment.
Test messages through common forwarders and mailing lists to confirm DKIM survives edits. Watch DMARC reports to confirm high DKIM pass and alignment rates across flows. Set DKIM alignment as the gate for passing and act now to make it universal.
Loosen Checks To Support Vendors
Relaxed alignment lets subdomains count as aligned, which helps when multiple vendors and hosts send for the same brand. Set both adkim and aspf to relaxed so small domain differences do not cause fails. Keep the DKIM d= domains and SPF return paths under the same organizational domain as the visible From address. Use naming rules for subdomains to keep sources clear and avoid overlap.
Watch aggregate reports to confirm that aligned passes rise across all sources after this change. Tighten to strict later only when every sender can sign and align exactly. Turn on relaxed alignment now to keep mail flowing while you prepare for reject.