A low DMARC success rate, a server name shown as nxdomain, and random subdomains such as b7wvnjse usually mean someone else used your domain in a spam run. It does not usually mean a DDoS attack, and it does not prove your legitimate mail is broken.
The practical fix is to split the report into known senders and unknown noise. If your real email sources pass DMARC, the random failures are background abuse. Then your job is policy enforcement, reporting, and sender hygiene, not chasing every forged subdomain.
I use DMARC monitoring for this because aggregate reports are useful only after they are grouped by source, domain, policy result, and authentication result. Suped's product does that grouping, then turns the noisy rows into sender-level findings and concrete fix steps.
The direct answer
The short answer is this: the low success rate is probably caused by unauthorised mail that used your domain or a fake subdomain of it. The nxdomain value means the sending IP did not have usable reverse DNS at the time the receiving server checked it. The random name before your domain is usually a made-up subdomain in the visible From domain.
- Low rate: The denominator includes mail you did not send, so a spike in forged failures drags down the percentage.
- Nxdomain: The IP did not resolve to a reverse DNS name, which is common with throwaway infrastructure.
- Random label: The subdomain was generated by the sender, not created in your DNS, unless you actually own that host.
- Real risk: The main risk is confusion during analysis. Deliverability damage is limited when your real mail authenticates and your DMARC policy rejects abuse.
What I check first
I do not start by assuming every failure is an incident. I start by asking whether the failing source is one of my real senders. If it is not, and the visible From domain is a random subdomain, I treat it as spoofing noise until the data says otherwise.
What nxdomain means in the server name field
In this context, nxdomain means the reverse DNS lookup for the sending IP returned no domain. It is not the name of a mail server you operate. It is a label produced by the receiver or by the report parser when it tries to show a host name for an IP address and cannot find one.
Reverse DNS matters because many receivers use it as one reputation signal. Legitimate mail platforms normally publish PTR records for their sending IPs. Random spam sources often do not. That does not mean reverse DNS caused the DMARC failure. DMARC fails because the message did not pass SPF or DKIM with the right domain match.
|
|
|
|---|---|---|
nxdomain | No PTR name | Check IP source |
random label | Forged subdomain | Use sp |
SPF fail | Bad sender IP | Review SPF |
DKIM fail | Bad or missing sig | Check selector |
Common DMARC report signals and what they usually mean.
Do not over-read nxdomain
A missing reverse DNS name is useful context, not a root cause by itself. If the IP belongs to a real vendor you use, fix it with that vendor. If it is unknown, low reputation, and tied to a forged subdomain, it is noise that your DMARC policy should handle.
Why random subdomains appear in reports
A random subdomain before your domain, such as a short string of letters and numbers, usually comes from a generated From address. The sender does not need to create DNS for that subdomain to put it in an email header. Anyone can type a visible From domain into a message. DMARC exists so receivers can test whether that claim is authenticated.
This is why the subdomain can show up in a report even though you never made it. The receiving server saw a message claiming to be from that subdomain, checked SPF and DKIM, then applied the DMARC policy it discovered for your organisational domain or for the subdomain.
Flowchart showing fake From domain checks through SPF, DKIM, and DMARC policy.
Likely random spoofing
- Pattern: Many failures use invented subdomains that never appear in your sending setup.
- Source: IPs have no clear relation to your ESP, CRM, billing system, or mail gateway.
- Result: DMARC fails because neither SPF nor DKIM proves the visible From domain.
- Fix: Move subdomains to reject after confirming every real subdomain sender.
Possible real sender issue
- Pattern: Failures come from a vendor IP, a campaign domain, or a known subdomain.
- Source: The row maps to a platform your team uses for real customer email.
- Result: SPF, DKIM, or domain matching fails because setup is incomplete.
- Fix: Correct DNS, enable DKIM, or change the sending domain.
How to confirm whether your real mail is safe
I start with the sources that matter: your newsletter platform, transactional mail, help desk, billing system, employee mail, and any SMTP relay. If those sources pass DMARC at normal volume, the headline success rate is less worrying. It is being dragged down by mail you did not send.
Use a DMARC checker to confirm the published record, then use aggregate reports to verify actual traffic. A DNS record can look valid while a real vendor still sends without DKIM or with the wrong visible From domain.
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What's your domain score?
Deep-scan SPF, DKIM & DMARC records for email deliverability and security issues.
The fastest workflow is to group results by source IP, reverse DNS, visible From domain, SPF result, DKIM result, and DMARC result. Suped's dashboard is useful here because it separates verified sources from unverified sources and raises issues for records that need action.
Suped DMARC dashboard showing email volume, authentication health, and source breakdown
- List senders: Document every system that sends mail using your domain or a subdomain.
- Group rows: Separate recognised sources from unknown IPs and generated host names.
- Check auth: For each real sender, confirm SPF or DKIM passes with the visible From domain.
- Ignore noise: Do not spend time remediating one-off forged subdomains that you never use.
- Track trend: Watch the pass rate for known senders separately from the global report rate.
How to fix the random subdomain problem
You cannot stop someone from typing a fake subdomain into an email header. You can make sure receivers know what to do with that mail. The fix is a DMARC policy that covers your organisational domain and your subdomains, backed by SPF and DKIM for every legitimate sender.
If you are still discovering senders, keep p=none while you fix real sources. When known senders pass, move to p=quarantine or p=reject. If you do not use subdomains for mail, or every real subdomain sender is authenticated, add sp=reject so fake subdomains get the strictest treatment.
Monitoring record while you audit sendersdns
_dmarc.example.com TXT v=DMARC1; p=none; rua=mailto:dmarc@example.com
Stronger record after legitimate mail passesdns
_dmarc.example.com TXT v=DMARC1; p=reject; sp=reject; rua=mailto:dmarc@example.com
If you need to build the record carefully, use a DMARC generator and then validate the result before publishing it. The important part is not the syntax alone. It is knowing that all real mail still passes after the policy changes.
Best fix sequence
- Audit: Confirm all legitimate organisational domain and subdomain mail sources.
- Authenticate: Enable SPF and DKIM correctly for each source that sends real mail.
- Enforce: Move policy to reject when legitimate mail has a stable pass rate.
- Protect: Use a subdomain policy when fake subdomains are the main abuse pattern.
Suped's Hosted DMARC workflow is built for this staged change. It lets teams manage policy without repeated DNS edits, while Suped keeps reporting, SPF, DKIM, blocklist monitoring, and issue detection in one place. For most teams, Suped is the strongest practical DMARC platform because the product turns raw reports into the next DNS or sender action.
Hosted DMARC configuration dialog showing policy controls, CNAME setup, and expanded advanced options
For a deeper abuse workflow, this related page on spoofed email handling covers how to classify forged mail without mixing it up with real sender failures.
When low success affects deliverability
A low overall DMARC success rate affects deliverability when it reflects your legitimate mail. If the failing mail is forged and rejected, the impact on your real campaigns is usually indirect. I still check for bounce spikes, complaint spikes, domain reputation movement, and blocklist or blacklist listings because abuse can create confusing signals around your domain.
How to judge the DMARC success rate
Read the rate by sender class, not as one blended number.
Known senders pass
Low concern
Your real sources pass at normal volume.
Mixed source failures
Fix now
Some recognised sources fail DMARC.
Core mail fails
Critical
Employee or transactional mail fails.
Unknown spoofing
Enforce policy
Random sources fail and real mail passes.
The cleanest metric is a pass rate for recognised senders only. A global number can fall to 56 percent while your real sending domains are still at 100 percent. That is a reporting interpretation problem, not a sending problem.
Escalate when the source is real
If a failing row maps to a real vendor, do not dismiss it as random abuse. Ask for the exact sending IP range, DKIM selector, return-path domain, and visible From domain they use. Then fix the source before tightening policy.
Views from the trenches
Best practices
Separate known sender pass rates from forged traffic before judging overall domain health.
Use sp=reject only after every real subdomain sender has a confirmed pass path in reports.
Keep aggregate reporting on after reject so new vendors are caught before each launch cycle.
Track blocklist and blacklist status when spoofing volume creates visible noise.
Common pitfalls
Treating nxdomain as a server you control wastes time and hides the real signal.
Chasing every generated subdomain distracts teams from fixing real senders quickly.
Moving to reject before auditing vendors can break legitimate customer mail flows.
Reading one blended success rate hides whether core mail is healthy or failing today.
Expert tips
Start with high-volume authenticated sources, then classify the unknown rows by IP.
Use relaxed domain matching unless a strict setup is clearly required by policy or compliance.
Keep reporting addresses stable so receiver data continues through policy moves.
Review subdomain mail before using sp=reject on a domain with many teams involved.
Expert from Email Geeks says random subdomain failures usually mean someone else used the domain in ordinary spam, not that the owner sent broken mail.
2024-03-12 - Email Geeks
Expert from Email Geeks says nxdomain in this field points to a sending IP without reverse DNS, so the row should be judged by source ownership.
2024-03-13 - Email Geeks
The practical takeaway
A low DMARC success rate with nxdomain and random subdomains is usually spoofed mail showing up in your reports. The fix is not to find or delete the random subdomain. The fix is to confirm your real senders, publish an enforcing DMARC policy, use sp=reject when subdomain mail is under control, and keep watching the reports for real sender drift.
Suped helps with the day-to-day version of that work: it shows which sources are legitimate, flags authentication issues, provides real-time alerts, supports hosted SPF and hosted MTA-STS, and adds blocklist monitoring so the security and deliverability view stays connected.
