How to interpret and resolve common Mailtester errors in your email campaigns

If you’re running email campaigns, you’ve probably run your messages through Mail-tester before hitting send. It spits out a score, flags problems, and sometimes makes you wonder if you should just give up and send carrier pigeons instead. This guide is for anyone who’s stared at those cryptic errors and thought, “What the heck do I actually need to fix?”

Let’s break down the common Mailtester errors, cut through the noise, and figure out what’s worth your time—and what isn’t.

1. Why Mailtester Scores Matter (But Aren’t Gospel)

Mail-tester checks your email for things that can trip up deliverability: spammy content, technical misconfigurations (like missing authentication records), and blacklists. Its score is useful, but not the last word. Real inbox placement depends on lots of things—your reputation, recipient engagement, and plain old luck.

Takeaway: Treat Mailtester as a helpful preflight check, not an absolute judge. A perfect 10/10 isn’t required. Aim for “good enough” so your emails land in the inbox, not the spam folder.

2. How to Use Mailtester (Without Losing Your Mind)

Before we dive into errors, here’s the smart way to use Mailtester:

  • Send a real test: Don’t just paste your message. Send from your actual sending address, through your real ESP or SMTP, to the address Mailtester gives you.
  • Check multiple times: Tweak, resend, and compare. One test can miss things.
  • Don’t chase 10/10: If you’re at 8 or 9 and the issues left are nitpicks, move on.

3. Decoding Common Mailtester Errors—And What To Do About Them

Let’s get real about what Mailtester reports, starting with the stuff that actually matters.

3.1 SPF, DKIM, and DMARC: The Big Three

a. SPF Record Not Found or Invalid

  • What it means: Your SPF (Sender Policy Framework) record is missing or misconfigured. This tells receiving servers who’s allowed to send email for your domain.
  • How to fix:
  • Log into your DNS settings.

  • Add or correct your SPF TXT record, e.g.:

    v=spf1 include:your-esp.com ~all

  • Use only one SPF record per domain. Multiple records = SPF fail.

  • Pro tip: Don’t just copy/paste from guides. Use your ESP’s documentation—they’ll tell you exactly what to include.

b. DKIM Signature Missing or Broken

  • What it means: Your email isn’t cryptographically signed, or the signature is broken. DKIM (DomainKeys Identified Mail) proves the email wasn’t tampered with.
  • How to fix:
  • Set up DKIM in your ESP. Usually, they’ll give you a DNS record to add.
  • Wait for DNS to propagate, then retest.
  • Worth fixing? Yes. Missing DKIM is a red flag for spam filters.

c. DMARC Not Set Up

  • What it means: You’re missing a DMARC (Domain-based Message Authentication, Reporting, and Conformance) policy. This tells other servers how to handle failed SPF/DKIM checks.
  • How to fix:

  • Add a DMARC TXT record to your DNS, e.g.:

    v=DMARC1; p=none; rua=mailto:you@yourdomain.com

  • “p=none” is safe for monitoring. Later you can tighten it.

  • Does it matter? Increasingly, yes. Some inbox providers now require DMARC.

3.2 Blacklists: When to Panic (and When to Ignore)

a. Listed on Blacklists (e.g., SORBS, Spamhaus)

  • What it means: Your sending IP or domain is on a known spam blacklist.
  • How to fix:
  • Check if this is your IP or your ESP’s shared IP.
  • If it’s yours, investigate why. Clean up compromised accounts.
  • For ESPs, contact support—they rotate IPs for this reason.
  • Request delisting, but only after you’re sure the issue’s fixed.
  • Ignore? Some obscure lists barely matter. But if you’re on Spamhaus, that’s serious.

3.3 SpamAssassin Warnings: Not All Spammy Words Are Created Equal

Mailtester runs your message through SpamAssassin and points out “spamminess.” Here’s what’s actually worth worrying about:

  • High scores (3+ points on a single rule): Fix these—they’ll hurt deliverability.
  • Minor nitpicks (<1 point): Don’t sweat every “buy now!” or exclamation point.
  • HTML issues: Broken HTML or missing plain text versions can trigger filters. Fix those.

What actually works:
- Avoid all-caps, too many images, or “too good to be true” offers. - Keep your lists clean. The best content won’t save you if you’re emailing dead or purchased lists.

3.4 Broken Links, Images, and Tracking Pixels

  • What it means: Links or images in your message don’t load, or your tracking pixel looks suspicious.
  • How to fix:
  • Double-check all URLs and image links.
  • Don’t use URL shorteners—they look spammy.
  • Tracking pixels are fine if legit, but avoid 1x1 transparent GIFs from sketchy sources.

3.5 Reverse DNS, HELO, and PTR Record Errors

  • What it means: Your sending server’s IP can’t be matched back to your domain (reverse DNS), or your server introduces itself with a generic name (HELO).
  • How to fix:
  • For most users on ESPs (Mailchimp, SendGrid, etc.), ignore these.
  • If you run your own server, set up proper PTR and HELO values.

Reality check: If you’re not running your own mail server, these errors are your ESP’s problem, not yours.

3.6 “You Are Not Fully Authenticated” or “Your Message Looks Like Spam”

  • What it means: One or more of SPF, DKIM, or DMARC is broken/missing, or your content is tripping spam filters.
  • How to fix:
  • First, fix authentication records (see above).
  • Next, tweak content:
    • Ditch spammy phrases.
    • Add a plain-text version.
    • Use real reply-to addresses.

4. Errors You Can Usually Ignore

Not everything Mailtester flags is urgent—or even actionable.

  • “No List-Unsubscribe header”: Nice to have, but not always critical. Some ESPs add it automatically.
  • Minor HTML warnings: If your email looks fine in major clients, move on.
  • Tiny SpamAssassin hits: Nobody gets zero. Focus on the big ones.

5. Troubleshooting Workflow: What To Fix First

  1. Authentication (SPF, DKIM, DMARC): These are the foundation. If they’re missing, fix them.
  2. Blacklist status: Only if it’s a major list or your own IP.
  3. Spammy content: Tweak obvious offenders, but don’t overthink every word.
  4. Broken links/images: Do a quick check.
  5. Retest: Send again, see if the score improves. Don’t chase perfection.

6. Pro Tips for Real-World Deliverability

  • Your domain’s reputation matters more than your Mailtester score.
  • List hygiene is king: Clean, engaged lists = better inboxing.
  • Test in the wild: Gmail, Outlook, Yahoo—don’t just trust Mailtester.
  • Monitor, don’t obsess: Check once in a while, but don’t let testing stop you from shipping.

Keep It Simple—and Keep Sending

Mailtester is a handy tool, but it’s just one piece of the puzzle. Fix what really matters, ignore the rest, and focus on sending useful emails to people who actually want them. If you’re in the green zone and your emails look good in real inboxes, you’re on the right track. Don’t let perfect be the enemy of done.