Most email teams test content before sending, checking links, proofing copy, and previewing on mobile. Few test deliverability before sending, even though a deliverability failure is far more damaging than a broken link.
A campaign that goes to spam generates zero revenue, erodes subscriber trust, and adds negative signals to your domain reputation. A campaign with a broken link in the footer generates some annoyance but still delivers and still converts.
Testing deliverability before sending requires specific tools and a specific sequence. This guide walks through every test worth running before a campaign goes live, what each test reveals, and what to do when a test returns a problem.
Why Pre-Send Testing Is Different from Post-Send Analysis
Post-send analysis tells you what happened. Pre-send testing influences what will happen.
When you check open rates and bounce rates after a campaign, you are measuring the outcome of sending. The damage from bounced addresses, spam reports, and reputation signals is already done. Analysis tells you what to do differently next time.
Pre-send testing catches problems before they generate any negative signals. A failed authentication test before sending means zero sends are affected. An authentication failure discovered through post-send bounce analysis means thousands of sends failed.
The time investment in pre-send testing is small compared to the cost of the problems it prevents. For large campaigns or high-stakes sends, it is the standard of care that serious email programmes apply.
Test 1: Authentication Verification
Run this test for every campaign from any new sending configuration, and monthly for established configurations.
Authentication is the most fundamental deliverability check. If authentication is broken, no other optimisation matters; every send is generating weaker reputation signals than it should.
How to Run It
Send a test email from your normal campaign workflow to a personal Gmail account you control. Open the email. Select “Show original” (the three dots menu in Gmail). In the raw header view, search for “Authentication-Results.”
Look for three lines:
- spf=pass
- dkim=pass
- dmarc=pass
All three should show “pass.” If any shows “fail” or “neutral,” you have an authentication problem to investigate before sending.
Alternative Tool: mail-tester.com
Visit mail-tester.com. Copy the test email address provided. Send your campaign email to that address from your normal sending workflow. Return to the site and view your score.
The score out of 10 includes authentication results (SPF, DKIM, DMARC), spam filter scoring, blacklist status, unsubscribe header presence, and HTML quality. A score below 8.5 warrants investigation.
What to Do If Authentication Fails
SPF fail: check mxtoolbox.com/spf.aspx for your domain. Is your ESP’s sending infrastructure included in your SPF record? Is the lookup count below 10?
DKIM fail: check your ESP’s authentication settings. Is DKIM enabled and configured with your domain’s key? Is the DNS record published?
DMARC fail with SPF and DKIM passing: this usually means alignment failure. Your DKIM signature domain does not match your From address domain. Enable custom domain DKIM signing in your ESP.
Test 2: Spam Score Assessment
Run this test for new email templates and for campaigns using content types you have not tested recently.
Spam score testing runs your email through spam filter rules, ts primarily SpamAssassin, and identifies which specific content patterns are adding to the score.
How to Run It
Use mail-tester.com (which includes SpamAssassin scoring) or a dedicated spam score tool. Send your campaign email to the provided test address and review the results.
SpamAssassin scores to understand:
- Below 1.0: clean. No meaningful content-related spam signal.
- 1.0 to 3.0: minor issues. For domains with High reputation, unlikely to affect inbox placement. Worth reviewing for rules you can easily fix.
- 3.0 to 5.0: moderate issues. May contribute to filtering at some providers, especially corporate email security gateways.
- Above 5.0: significant issues. These scores can affect inbox placement even for established domains.
What the Score Reveals
Mail-tester.com and similar tools show you exactly which rules triggered and how many points each added. Common triggers include: all-caps in the subject line, certain HTML formatting patterns, missing plain text alternative, and link-to-text ratio issues.
Fix the highest-point-value triggers first. A single rule adding 2.0 points is more important to fix than five rules each adding 0.2 points.
Test 3: Inbox Placement Testing
Run this test before any campaign above 50,000 sends, and before the first send of any new campaign type or from a new sending domain.
Inbox placement testing uses a seed list,t a set of real email addresses at major inbox providers, to show you where your email actually lands.
How It Works
Services like GlockApps, Mail Reach, and Litmus place test email addresses at Gmail, Outlook, Yahoo, Apple Mail, and other providers. You send your campaign to those test addresses. The service reports where your email landed: inbox, Promotions, spam, or (in some cases) whether it was not delivered at all.
What the Results Tell You
If you see “inbox” across most providers, your deliverability is strong for this campaign template and sending domain.
If you see “spam” at a specific provider, you have a provider-specific reputation or content issue affecting that inbox provider.
If you see “Promotions” at Gmail, your email is correctly classified as promotional content. This is not necessarily a problem;m many Gmail users check Promotions regularly, but it explains lower open rates from Gmail recipients.
GlockApps: The Recommended Tool
GlockApps is the most widely used dedicated inbox placement testing tool. It covers more inbox providers than most alternatives, provides spam filter analysis, and offers ongoing monitoring (not just one-off tests). Plans start around $19 per month for limited tests.
Test 4: Domain Reputation Check
Run this test weekly as part of your monitoring routine, and specifically before any major campaign.
Google Postmaster Tools
Log in to postmaster.google.com. Check the Domain Reputation chart for your sending domain. It should show High.
If it shows Medium, investigate what recent campaigns may have generated elevated bounce or complaint signals. Address the source before your next send.
If it shows Low or Bad, do not send the planned campaign to the full list. Implement the reputation recovery protocol first.
Sender Score
Visit senderscore.org and enter your sending IP address. A score above 80 is healthy. A score below 70 warrants investigation. A sudden drop from previous weeks signals a recent reputation event to identify.
Test 5: Blacklist Status Check
Run this test monthly and before any major campaign.
How to Run It
Visit mxtoolbox.com/blacklists.aspx. Enter your sending domain. Then enter your primary sending IP address. Check both.
The tool queries over 100 blocklists simultaneously. A clean result shows no listings. Any listing, particularly on Spamhaus SBL, Spamhaus DBL, SURBL, or Barracuda, da requires immediate action.
If You Find a Listing
Do not send the planned campaign until the listing is addressed. A blacklist listing means some proportion of your email is being blocked or heavily filtered at providers that integrate that blocklist. Sending during an active listing adds more negative signals and makes delisting harder to obtain.
Go to the specific blocklist’s website. Look up your IP or domain entry. Follow their delisting process. Spamhaus, the most impactful blocklist, requires you to demonstrate the root cause has been fixed before they process removal.
Test 6: List Quality Verification
Run this test before any campaign to a segment not recently verified or to an imported list.
Email verification confirms that the addresses in your send list are deliverable. This is the only test that directly prevents bounce signals before they occur.
When Verification Is Essential
Before any re-engagement campaign: these segments have the highest concentration of invalid addresses.
Before any import is deployed in a campaign: external lists have unknown quality characteristics.
Before any campaign to a segment inactive for more than 60 days: decay accumulates during periods of no sending.
Before any major campaign (100,000+ sends): the cost of a bounce spike at this scale justifies verification regardless of recent history.
Running the Test
Upload your campaign audience to your email verification provider. Classify results and suppress invalid, disposable, and role-based addresses before importing the verified-clean audience to your ESP for the campaign.
Test 7: Preview Text and Subject Line Review
Run this test for every campaign as part of standard content QA.
This is not a technical deliverability test; instead, it is a content review that affects engagement signals. Engagement signals affect domain reputation. Low-quality engagement is a slow-moving deliverability problem.
What to Check
Subject line: Does it avoid all-caps words? Does it match what the email delivers? Is it under 60 characters to avoid truncation on mobile?
Preview text: Is it configured deliberately (not accidentally showing “View this email in your browser”)? Does it extend the subject line’s value rather than repeating it?
Unsubscribe link: Is it clearly visible in the email body? Does the List-Unsubscribe-Post header appear in the email headers (confirming one-click unsubscribe is active)?
Sender name and From address: Will recipients recognise who this is from? A From name they do not recognise increases spam reports.
Building a Pre-Send Testing Workflow
A pre-send testing workflow makes these tests systematic rather than optional.
For major campaigns (above 100,000 sends), run all seven tests. Build a simple checklist template, a shared document, or a project management task that requires each test to be marked complete before a campaign can be approved for deployment.
For standard weekly campaigns to established audiences, run a shortened version:
- Authentication check (monthly, not per campaign)
- Domain reputation check in Postmaster Tools (weekly)
- Blacklist check (monthly)
- List quality: verify the segment if it includes any inactive contacts or new imports
For routine sends to small, recently-verified engaged audiences, the minimum is a Postmaster Tools reputation check and a hard bounce rate review from the previous campaign.
Assigning Ownership
Pre-send testing only works consistently when someone owns it. Assign one person on your team as the deliverability check owner for every campaign. Their sign-off is required before the campaign deploys. Without named ownership, the checks become optional and eventually stop happening.
Key Takeaways
- Pre-send testing catches deliverability problems before they generate negative signals. Post-send analysis tells you what happened; pre-send testing influences what will happen.
- Seven tests to run before major campaigns: authentication verification, spam score assessment, inbox placement testing, domain reputation check, blacklist status check, list quality verification, and subject line/preview text review.
- Run authentication verification for every new sending configuration and monthly for established ones. All three records (SPF, DKIM, DMARC) should show “pass” in Gmail’s email headers.
- Run inbox placement testing (GlockApps or similar) before campaigns above 50,000 sends. It is the only way to know where your email actually lands before you commit to sending to your full audience.
- Build the tests into a named workflow with assigned ownership. Without named responsibility, pre-send testing becomes optional and eventually disappears.
- The minimum routine for regular campaigns: check Postmaster Tools domain reputation and review the previous campaign’s hard bounce rate. Scale up to full testing for major or high-risk sends.
Frequently Asked Questions
Reduce frequency first. If engagement doesn’t improve after 60–90 days, consider suppressing them.
Yes, but it can improve engagement rates, deliverability, and overall list quality.
Set clear expectations, segment by interests, control sending frequency, and regularly refresh your content.
After a consistent period of low engagement, especially when reduced frequency and re-engagement campaigns have failed.
Not necessarily. A smaller list of genuinely engaged subscribers can produce better engagement and deliverability than a larger inactive list.
Conclusion
Pre-send testing is the practice that separates reactive email programmes from proactive ones. It costs 2 to 3 hours before a major campaign. It prevents reputation damage that costs weeks to repair.
Start with authentication verification; run it this week if you have not recently. Then add domain reputation monitoring as a weekly habit. For major campaigns, graduate to the full seven-test suite.
Every test catches a different problem. Together, they give you confidence that your campaign will reach the inbox before you commit to sending to your entire audience.
