How to Run an Email Deliverability Test Before You Send

Your campaign is ready, the links work, and the test email arrived in your own inbox. Before sending to the whole list, there's another useful check: where will that email land at the providers your customers use?

An email deliverability test helps answer that question by sending the finished message through your normal sending route to controlled test recipients. MailSlurp's inbox placement test reports whether the copies reach inbox, Promotions, spam, or remain unobserved during the test window. You can then investigate a specific problem before the campaign goes out.

What does a deliverability test actually prove?

The word "delivered" can describe several different events. Let's separate them so a reassuring send report doesn't hide an unfinished check.

Send accepted: your email service accepted work to send the message. Follow its delivery, bounce or rejection result for each recipient.

Message received: a controlled MailSlurp inbox received the copy, with content and headers you can inspect. You can now check the link, code or content needed by the user journey.

Provider placement observed: a seed mailbox reports inbox, Promotions or spam for the tested copy. Compare completed results for equivalent provider samples to see whether a repair changed the outcome.

A seed is a controlled mailbox used for testing. A placement run samples available providers; it doesn't predict the folder for every subscriber. Your campaign's ordinary bounce, complaint and engagement reports still matter alongside the test.

For signup, password reset and other application messages, also finish the action in the email. A reset message arriving is only part of a successful password reset. Use expectation-based deliverability tests for message assertions and the test email address guide for the complete application journey.

Start with the campaign you plan to send

Use the published campaign revision, the usual From address and the same email service provider, or ESP, that will send to your audience. An email sent from a personal mailbox won't exercise a campaign platform's authentication, tracking links or sending infrastructure.

Keep the subject, links, images and relevant personalization representative of the real send. If the platform has a special "send test" mode, check whether it uses the same route and content as the published campaign. The provider walkthroughs below show the appropriate workflow.

Choose the provider coverage that matters to your audience from the options available for the run. Consumer and business inboxes can behave differently. A successful Gmail sample doesn't settle what will happen at Microsoft 365.

Run the placement test in MailSlurp

For a manual campaign test:

1. Open Inbox placement in MailSlurp and create a new test.
2. Copy the exact marker and every seed address supplied for that run.
3. Add the marker to the campaign subject or body. Keep it intact when editing or publishing the email.
4. Put the current seed addresses into a dedicated test audience in your ESP. Confirm that the send targets that audience rather than your normal customer list.
5. Send the finished campaign through the provider and check its recipient report for exclusions or rejected sends.
6. Return to MailSlurp and let the observation window finish. Review the provider rows, Content issues and Domain issues.

Each run has its own marker and seed set. Reusing yesterday's addresses or marker can leave today's result incomplete. The email seed-list guide explains audience isolation and how to reconcile recipients that an ESP excluded.

Follow the steps for your sending platform: Mailchimp campaigns use a dedicated tagged segment; Klaviyo campaigns use a consented test list; HubSpot marketing emails need the correct marketing-contact setup. The placement documentation lists the other provider guides.

For recurring tests through a supported sending connection, use placement automation and reports. Follow that connection's setup and completion checks so each run tests the intended sender.

Read the provider rows before deciding what to fix

A percentage is easier to interpret after you understand which messages were included. Start with the completion state and the individual provider outcomes.

• Inbox or Primary: the copy reached the provider's main inbox experience. Compare it with the same provider in the previous completed run.
• Promotions: the provider delivered the copy to a promotional category. Treat it as delivered and assess it in the context of the message.
• Spam or junk: the provider placed the copy in its unwanted-mail folder. Inspect authentication and the run's content/domain findings.
• Pending: the check is still in progress. Wait for completion before treating it as a final result.
• Not received: the message wasn't observed at that seed during the test window. Check the ESP send report, recipient inclusion and exact marker.

Promotions is different from spam. A product announcement in Promotions and a missing password reset require different investigations. Likewise, "not received" doesn't identify a cause by itself: the ESP might have excluded the recipient, the send might have failed, or the copy might not have been observed in time.

Use the inbox placement rate guide when comparing percentages. It explains how Promotions, missing results and changes in provider coverage affect the calculation. There isn't a universal passing percentage that makes every campaign safe to send.

Investigate the problem the result shows

A message is missing

First compare the current seed list with the provider's actual recipient report. An imported contact isn't necessarily an included recipient: list membership, consent, suppression and campaign filters can affect the final audience.

For an included recipient, look for the provider's send outcome and any rejection or deferral. Then check that the published email contained the current marker. If the run is still pending, let it complete before starting a second test; overlapping runs can make it harder to tell which send you're investigating.

The email arrives in spam

Inspect the received headers with the email header analyzer. SPF checks which servers may send for an envelope domain; DKIM supplies a domain signature; DMARC checks whether a passing SPF or DKIM identity aligns with the visible From domain. A DNS record existing is different from the delivered message passing the relevant checks.

Use those findings alongside MailSlurp's content and domain issues to choose a repair. A broken authentication setup needs a sender fix. A repeated problem at one provider needs a provider-specific investigation, such as the Outlook junk-email walkthrough.

Avoid changing the sender, subject, template and volume all at once. A focused correction makes the next result easier to interpret. If you're preparing a new or recovering sender, follow the email warmup guide separately; a warmup program doesn't replace testing the actual campaign.

Placement is good, but the email is hard to use

Now check the experience after arrival. A button can disappear in dark mode, an image can fail to load, or an Outlook layout can make the offer difficult to read even when the message reaches the inbox.

Device previews show the delivered email across supported clients and devices. Email Audit checks content problems such as broken links and images. These checks complement placement: each answers a different question about the same campaign.

Make the next test comparable

After a repair, create a new run and send the revised campaign with its new marker and seed addresses. Compare completed outcomes for matching providers and coverage. If one run has fewer available samples, keep that difference visible instead of interpreting a larger percentage as automatic improvement.

Keep the earlier result beside the new one and note the actual change: for example, corrected DKIM signing or an ESP audience filter that excluded seeds. Compare the campaign revision, sender, sending route, provider counts and completion state. Those details help you tell whether the repair worked or whether the two tests exercised different conditions.

Before releasing the campaign, resolve any missing-message investigation, review repeated spam outcomes at important providers, and finish the content and device checks. Use your normal customer-send reports to monitor what happens after launch.

Common questions

Can I test email deliverability without writing code?

Yes. Create a placement test in MailSlurp, copy the marker and seed recipients into your campaign platform, send to the isolated test audience, and review the provider results. Use your ESP's walkthrough for the audience and sending steps.

Is a spam score the same as inbox placement?

A spam score summarizes the checks that produced it. Inbox placement observes the folder outcome for tested copies at providers. A clean content check is useful, but the placement run shows what happened to the sent email.

When should I repeat the test?

Repeat after meaningful changes to the campaign, sending domain, authentication or provider. Use comparable completed runs when investigating a problem. Testing the revised campaign is more useful than repeatedly sending an unchanged sample without a specific question.

Can I use this for transactional email?

Yes. Test a representative message through its actual sending route. For an OTP, reset or verification link, add an application test that consumes the received message and proves the user can complete the action.

Ready to check your next send? Run an inbox placement test with MailSlurp.