How to optimize your email headers using the Mailtester analysis tool

If your emails keep vanishing into spam folders, you're not alone. Email deliverability is a pain—especially when you’re doing everything “by the book” and still end up in the junk pile. The culprit is often hiding in your email headers. This guide is for anyone who’s frustrated by deliverability issues and wants real, practical steps using the Mail-tester tool. No magic bullets—just clear advice, so you can get your emails where they belong: the inbox.

Why Email Headers Matter (and What Most People Get Wrong)

Email headers are the technical bits at the top of every email. They tell mail servers who sent the message, where it’s from, and whether it’s trustworthy. Most of the time, people ignore them, but spam filters sure don’t.

Here’s the tough truth: You can write the world’s best newsletter, but if your headers are off—missing authentication, showing the wrong sender, or riddled with weird formatting—your message could get trashed before anyone sees it.

Common header screw-ups:

  • No DKIM or SPF records (or set up wrong)
  • Mismatched sender domains (e.g., “From” says one thing, but “Return-Path” says another)
  • Weird or missing “List-Unsubscribe” links
  • Oddball message IDs or formatting errors

You don’t have to be a server admin to fix this stuff. But you do need to see what’s actually going out—and that’s where Mail-tester comes in.

Step 1: Send an Email to Mail-tester

First, you need to see what your email really looks like to a spam filter. Mail-tester’s free tool does just that.

How to use it:

  1. Go to Mail-tester.
  2. You’ll see a weird-looking email address. Copy it.
  3. Send your typical marketing, transactional, or newsletter email to that address—from your real system, not just a test draft.

Pro tip:
Send an actual campaign or transactional email, not just a blank “hello.” The goal is to mimic what your real recipients get, headers and all.

Step 2: Read Your Mail-tester Report

Once your email lands, click “Then check your score” on Mail-tester. You’ll get a report card, usually out of 10.

Pay attention to:

  • Authentication: SPF, DKIM, and DMARC results
  • Blacklists: Is your sending server on any blocklists?
  • Header Analysis: Any warnings about missing or weird headers?
  • SpamAssassin Score: Are you getting flagged for spammy content or formatting?

Don’t obsess over the number.
A perfect 10/10 isn’t required. You want to fix the big red flags, not chase every minor warning.

Step 3: Fix Authentication Issues (SPF, DKIM, DMARC)

Most deliverability problems start here. If Mail-tester says you’re missing SPF, DKIM, or DMARC, take it seriously.

What to do:

  • SPF:
    Add or update your domain’s SPF record. This is a DNS TXT record that tells receiving servers which mail servers are allowed to send for your domain.

  • DKIM:
    Set up DKIM signing in your email system. This adds a digital signature to your emails, proving they weren’t tampered with.

  • DMARC:
    DMARC ties SPF and DKIM together and gives instructions on what to do with failed messages.

Resources:
- Most major sending platforms (Mailchimp, SendGrid, etc.) have step-by-step guides for adding these records. - If you use your own mail server, you’ll need access to your DNS settings.

Pro tip:
These changes can take time to propagate—sometimes up to 48 hours.

What not to do:
Don’t try to “trick” these systems. Don’t use random DKIM keys or generic SPF records you found online. Use the ones generated for your domain and provider.

Step 4: Clean Up Your Sender Information

Mail-tester will flag mismatches in your sender details—these are easy to fix, but easy to overlook.

Check for:

  • Does your “From” address match your domain and SPF/DKIM setup?
  • Is your “Return-Path” address set correctly?
  • Are you using a real reply-to address?

If you see warnings like: - “From domain does not match SPF domain” - “Return-Path is different from sending domain”

You probably need to: - Adjust your sending platform’s settings so the email comes from an address that matches your authenticated domain. - Avoid using free webmail addresses (like Gmail or Yahoo) as your sender if you’re sending from your own domain.

Pro tip:
If you’re on a platform that sends mail “on your behalf” (like a CRM or ESP), make sure you’ve authenticated your domain, not just their default.

Step 5: Add Helpful Headers (and Ignore the Useless Ones)

Some headers actually help with deliverability or user experience. Others are just noise.

Worth adding:

  • List-Unsubscribe:
    Lets people unsubscribe easily (and some inboxes show a handy “unsubscribe” link). Most email platforms add this automatically.

  • Message-ID:
    Should be unique per message. Again, most platforms handle this for you.

  • Precedence: bulk
    For newsletters and promos, adding Precedence: bulk signals this isn’t a personal email. Helps with filtering, sometimes.

Don’t bother with:

  • X-Mailer or X-Originating-IP:
    These don’t help with deliverability, and can sometimes hurt if they show you’re using an old or generic system.

  • Weird custom headers:
    Unless you know why you need them, skip them.

Pro tip:
If you’re using a modern ESP or transactional service, you probably don’t need to mess with headers manually. But check Mail-tester to make sure they’re set up right.

Step 6: Watch Out for Formatting and Encoding Gotchas

Mail-tester will sometimes flag formatting issues in your headers or body that can trigger spam filters.

Common mistakes:

  • Non-UTF-8 encoding (use UTF-8 unless you have a good reason not to)
  • Broken line breaks or missing header fields
  • Very long subject lines or weird characters

How to avoid:

  • Stick to plain ASCII or UTF-8 in your headers.
  • Keep your subject lines reasonable (under 78 characters is a safe bet).
  • Don’t copy-paste from Word or Google Docs into your email tool—this can add invisible junk.

Step 7: Ignore the Noise, Focus on What Matters

Mail-tester sometimes flags things that don’t matter much in the real world. If you see a yellow warning about “No rDNS” or “MISSING_MID,” don’t panic.

What to ignore:

  • Minor header warnings that don’t affect authentication or sender reputation
  • Warnings about images not having alt text (unless you care a lot about accessibility)
  • Slight deductions for not having a perfect “List-Unsubscribe” format

What to fix:

  • Anything in red, especially SPF/DKIM/DMARC failures or blacklisting

If your emails are landing in the inbox, don’t sweat the small stuff.

Step 8: Test Again (and Regularly)

Once you’ve made changes, send another email to Mail-tester. Deliverability isn’t “set-it-and-forget-it”—DNS records can break, platforms update their defaults, and ISPs change their filters.

Schedule regular tests:

  • After any major change to your sending system
  • Once a month, just to be safe
  • If you start seeing a drop in open rates (could be a sign of landing in spam)

Pro tip:
Use different content and subject lines in your tests. Sometimes one subject line or phrase can trigger a spam filter even if your headers are perfect.

Keeping it Simple

Email deliverability isn’t rocket science, but it’s easy to get lost chasing tiny tweaks. Use Mail-tester to catch the big problems—authentication, sender mismatches, missing unsubscribe links—and ignore the noise. Make one change at a time, test, and move on. Spend your energy on writing good emails, not obsessing over a perfect score.

You don’t need to be a sysadmin to fix your headers—you just need to pay attention to what Mail-tester tells you, and keep things straightforward. Inbox, here you come.