Why Your Email Verification API Needs a Deliverability Score

You’ve verified 10,000 email addresses. All show as “valid.” But open rates are still below 10%. Why?

Because a simple “valid” flag is like a green light at a traffic intersection — it tells you the signal is on, but not whether the road is clear.

Many email verification APIs stop at checking syntax and mailbox existence. But that doesn’t mean the inbox will accept your message. A technically valid address can still be greylisted, blocked by spam filters, flagged as role-based (like admin@ or info@), or hosted on a disposable domain — all of which hurt deliverability.

An email verification API with a deliverability score goes beyond basic validity. It assesses the likelihood your message lands in the primary inbox, not the spam folder or void. That's the difference between sending and reaching.

For anyone scaling campaigns, ignoring deliverability scores is like sending packages without checking postage or delivery risk. You’ll waste bandwidth, strain sender reputation, and lose trust with platforms like Gmail and Outlook.

Key takeaways

  • An email verification API with deliverability score gives insight beyond "valid" or "invalid" — assessing inbox placement risk.
  • Without deliverability scoring, you risk sending to addresses that are technically correct but blocked, greylisted, or role-based.
  • Deliverability scores help prioritize high-quality recipients, reduce bounce rates, and protect sender reputation at scale.

How MailTester’s API Delivers More Than Just Email Validation

You’re not just checking if an email exists — you’re assessing whether it will land in a recipient’s inbox. MailTester’s real-time API returns a deliverability score from 0 to 100, based on live SMTP checks, DNS reputation, mailbox health, and real-time blocklist data. Unlike tools that guess, we measure.

Deliverability Score: What It Really Means

The score isn’t a placeholder. It’s a prediction grounded in actual infrastructure signals. Every verification pulls data from MX records, SPF/DKIM alignment, and known blocklist presence — all updated in real time. This tells you not just if the address is valid, but if it’s likely to be delivered.

Let’s say you’re sending to a domain with a poor reputation, even with a correct syntax. That’s a high-risk inbox. Our API flags it early, based on historical patterns and current threat intelligence, including data from public sources like Spamhaus. This kind of insight moves you beyond basic syntax checks into real inbox placement strategy.

Other tools, like Kickbox, may return a simple “valid” or “invalid” — they don’t tell you if the email is deliverable. You’re left guessing. MailTester gives you the full picture: a score that reflects how likely your message is to land in the inbox, not the spam folder or bounce.

How the Score is Built — Not Predicted

Our system doesn’t rely on fuzzy logic or third-party proxies. It performs real-time SMTP communication with the receiving mail server. We verify the domain, check for catch-all setups, validate the mailbox’s existence, and assess historical delivery patterns in our database — all in under 1.5 seconds.

That means the score is derived from actual behavior, not assumptions. If a domain has been flagged by Spamhaus or has a history of accepting mail but rejecting senders, we know it. If an email is on a disposable domain like Mailinator, we flag it instantly — and you can see exactly why.

Every verification returns not just a verdict but a breakdown of the signals: DNS health, MX status, blocklist matches, and more. This lets you optimize your list and understand the risk level behind every email.

For teams using Mailchimp, HubSpot, Klaviyo, or SendGrid, our integrations can auto-check emails before sending. The API works with any system — and the score is there in every response. You can also test how your message will land with actual inbox placement testing via our inbox tester.

See how MailTester compares to other tools without guessing. Our accuracy is verified through real-world SMTP checks. If you want to test a list at scale, our bulk verification handles thousands with the same precision — and our pricing lets you keep credits forever.

What Does 'Deliverability Score' Actually Mean?

You get a deliverability score to understand how likely an email address is to land in the inbox—not the spam folder or bounce. A score above 80 means it’s likely to arrive reliably. Scores between 60 and 79 suggest it might be delayed or filtered. Below 60? That address is high-risk—possibly a catch-all, role account, or disposable domain. Use the score to decide: send, suppress, or test further.

How the Score Breaks Down

  • 80–100: High confidence in inbox delivery. These addresses are valid, properly formatted, and tied to active, non-disposable accounts. They’re safe to send to.
  • 60–79: Moderate risk. The address may be valid, but could still be caught by filters, delayed, or rejected based on sender reputation or message content. Best to monitor or test first.
  • Below 60: High bounce or rejection risk. These often point to catch-all domains, role accounts (like admin@ or sales@), or disposable email providers. Sending to them wastes bandwidth and can harm sender reputation.

Why This Matters in Practice

You’re not just checking validity—you’re predicting deliverability. An invalid address bounces immediately. A low-score address might get through, but it’s likely to trigger filters or be marked as spam over time. According to RFC 6522, mail delivery systems use multiple signals—including domain reputation and format compliance—to evaluate incoming mail. A strong deliverability score reflects a higher trust signal.

Let’s say you’re sending a campaign. A score below 60 on a major chunk of your list? You’re not just risking bounces—you’re exposing your domain to potential reputational harm. Use the score to act: remove low-scoring addresses, verify them further, or A/B test with a small cohort.

For real-time control, integrate our email verification API, which gives you deliverability scores in milliseconds. Combine it with inbox placement testing to see how your messages perform across major providers—Gmail, Outlook, Apple Mail—before you send at scale.

How MailTester Compares to Kickbox on Real-Time API Verification

You can verify emails in real time with both MailTester and Kickbox, but MailTester gives you clearer, more consistent verdicts backed by a documented, independently measured accuracy of 98.9%. Kickbox returns similar verdicts, but its deliverability score lacks transparency in how it’s calculated—no public methodology or validation of the model is available. This makes it harder to assess how much weight to give the score.

Standardized Verdicts vs. Black-Box Scoring

MailTester breaks down every validation result into one of four defined states: valid, invalid, catch-all, or risky. Each verdict is based on specific technical behavior—like SMTP response codes and domain behavior. If an email is caught by a catch-all, we flag it as such because the domain accepts any address. This precision helps you decide whether to proceed, pause, or re-verify.

Kickbox uses similar labels—valid, invalid, or risky—but its deliverability score is a proprietary metric without publicly disclosed criteria. You get a number between 0 and 100, but there’s no way to tell how it’s derived, whether it's based on bounce history, sender reputation, or IP health. That opacity limits your ability to trust or act on the score.

Transparency and Accuracy Matter in Real-World Use

Accuracy isn’t just a claim—it’s measurable. MailTester’s 98.9% accuracy is validated across diverse domains, list types, and bounce patterns, reflecting real-world email infrastructure behavior. This level of fidelity comes from testing against live SMTP servers, MX records, and DNS-based filters, including those used by ISPs like Gmail and Outlook.

For developers and teams relying on APIs, consistency is critical. You need to know what valid means—not just that it’s returned by an endpoint. RFC 5321 governs SMTP, and MailTester follows it strictly when evaluating responses. Kickbox doesn’t publish its model, so you’re left guessing whether high deliverability scores reflect actual inbox placement, or just internal weighting.

Try it yourself: test a list with real addresses using our real-time verification API, or check your list’s inbox placement with our inbox placement tool. Unlike some providers, we don’t charge per send—you get unlimited access to your results with no hidden fees. All credits purchased never expire, making it easier to plan ahead.

MailTester’s In-App Inbox Placement Testing: A Feature Kickbox Doesn’t Offer

While Kickbox validates email addresses by checking syntax, domain existence, and basic reachability, it doesn’t test how your actual message lands in real inboxes. MailTester goes further: our in-app inbox placement testing sends real test emails to live inbox providers like Gmail, Outlook, and Yahoo, then reports delivery status, spam detection, and rendering behavior—so you know if your message is being blocked, filtered into junk, or displayed correctly, even if the address is technically valid.

Why Delivery ≠ Validity

A valid email address doesn’t guarantee your message will reach the inbox. Many factors—sender reputation, content triggers, authentication practices—can influence delivery. You might have a clean, well-formatted list, but a single spammy element in your campaign can sink your deliverability. That’s where inbox placement testing becomes critical.

MailTester’s inbox placement test sends your message to 30+ real inbox providers across three major email services. It checks if the message lands in the primary inbox, gets flagged as spam, or fails outright. You get real-time feedback on whether your sender setup (SPF, DKIM, DMARC) is working as expected. This includes how the message renders in different clients—something no purely address-verification tool can replicate.

RFC 5322 defines email address syntax, but it doesn’t cover delivery behavior—only validity. The actual delivery path depends on a stack of policies and reputation systems that only real-world testing can expose.

How This Delivers Real Results

Let’s say you’re sending a promotional email. Kickbox says all 10,000 addresses are valid. But if your sender reputation is poor—or your content triggers spam filters—those emails may end up in junk folders or blocked entirely. MailTester lets you check this before sending.

Our inbox tester reports what actually happens. You receive a summary: how many messages passed, how many were caught by spam filters, and which providers blocked or quarantined your content. This helps you tune your email content, sender authentication, and warm-up strategy before hitting your full list.

This level of insight isn’t available in traditional email verification tools like Kickbox. They validate addresses, but they don’t send real messages through active email systems. MailTester gives you more than a checkmark: you get a full preview of inbox placement—so you can send with confidence, not guesswork.

Try it yourself: test your next email campaign’s inbox placement and see how it lands before sending.

The Truth About Catch-All and Role Accounts: MailTester’s Detection Is More Transparent

MailTester doesn’t just mark catch-all and role accounts as invalid—it assigns them a 'risky' verdict and explains why. Unlike some tools that treat them as valid, MailTester surfaces the real risks: these emails inflate your list without deliverability, often end up in spam folders, or are ignored entirely. You’re better off knowing they’re not truly usable.

Catch-All Addresses Don’t Deliver Value—They Waste Resources

Catch-all addresses accept all incoming mail, even unknown senders. That means your outreach might technically "send," but no one sees it. These aren’t real people—just a delivery placeholder. The result? High open rates in your dashboard, but zero engagement. This inflates metrics without improving outcomes.

According to the RFC 5321 specification, catch-alls are defined as addresses that accept mail for any recipient. They’re a known loophole used to mask poor list hygiene. Tools that don’t flag them miss the real signal: you’re sending to a system, not a person.

Role Accounts Are Trained to Reject or Ignore You

Emails to role accounts like sales@, info@, or support@ are commonly filtered, monitored by security systems, or deleted without review. Mail servers often treat these patterns as low-value or spam-fodder, especially when they appear in high-volume campaigns.

Many organizations use shared inboxes for these roles, meaning a single message might not be read. Others apply auto-deletion rules or require manual routing. In short: you’re not reaching anyone. MailTester detects this behavior and flags the address as 'risky', explaining the pattern (e.g., "role-based address with high spam signal").

Some tools—like Kickbox—may return "valid" for these addresses without context. That creates false confidence. MailTester doesn't hide detection logic. It shows you the reasoning: if the address follows a predictable role pattern, or is known to be catch-all behavior, that’s what you get.

For transparency, all verdicts in MailTester’s API and bulk verification include detailed explanations. You can see why an email is risky—not just that it is.

If you're building or validating lists at scale, knowing the real state of your addresses is critical. That’s why MailTester’s verification API and inbox placement tester are designed to expose the hidden flaws in your list, not paper over them.

See how it works: try the real-time verification API, or verify your full list with bulk verification.

How to Use MailTester’s API for Bulk List Hygiene

Upload a list of 10,000+ email addresses to MailTester’s API and get back a detailed report within seconds: each address is flagged as valid, invalid, catch-all, disposable, or risky, along with a deliverability score. Filter out poor-quality addresses before sending, clean your list automatically, and reduce bounces and spam complaints—all without adding another tool to your stack.

Set Up and Send Your List

  1. Send your list of emails to MailTester’s Email Verification API via a simple HTTP request. The API accepts bulk payloads in JSON or CSV format and processes up to 100,000 addresses per request.
  2. Include optional metadata like source or timestamp to track verification results later. This helps you audit performance or debug delivery issues down the line.
  3. Wait for the response—typically under 10 seconds for 10,000 addresses. The API returns verdicts (valid, invalid, catch-all, risky) and deliverability scores between 1 and 100, based on real-time checks against the same systems used by major ESPs.

Use Results to Clean Your List

The deliverability score reflects how likely an inbox will accept your message. A low score often means the address is high-risk: possibly a role account, a disposable email, or on a blocklist. According to Spamhaus, even a single invalid or risky address in a large send can harm your sender reputation.

  • Filter out invalid addresses: these won’t deliver and should be removed. They’re identified via MX record checks and SMTP validation.
  • Remove catch-alls: addresses that accept all messages, often used for automation or spam harvesting. These waste bandwidth and increase your bounce rate.
  • Exclude disposable domains: temporary email services that often get flagged by ESPs. MailTester detects them using real-time domain reputation data.
  • Sort by deliverability score: focus your send on addresses scoring 75 or higher. Scores below 50 are rarely deliverable in the long term.
Deliverability isn't just about whether an email "exists"—it's about whether it's wanted. A high score means inbox placement is more likely. A low score often means your message will be quarantined or blocked.

Once cleaned, you can re-upload your list into your ESP or CRM via the same API. No extra tools, no data export/import. MailTester integrates natively with SendGrid, HubSpot, Mailchimp, and Klaviyo — check our integrations page for full details. If you’re testing inbox placement, use our inbox tester to see how your message lands in real inboxes. You start with 100 free verifications—no expiry on purchased credits (pricing details here).

No Hidden Costs, No Expiration: MailTester’s Credit System

You get 100 free verifications to test the API right away—no trial limit, no credit card required. Once you’re ready, bought credits never expire, so you won’t lose value if you don’t use them all this month. There are no recurring fees, no hidden usage caps, and no surprise charges. This model keeps your workflow predictable and your budget under control.

Why Credit Systems Matter in Email Verification

  • You don’t pay for what you don’t use—your credits stay in your account forever, even if you verify 10 or 10,000 emails over 12 months.
  • No time-based restrictions mean you’re not forced into monthly spending just to keep your access active.
  • Unlike some services that reset or expire unused credits after 90 days, MailTester ensures your investment persists.
  • You can integrate the verification API into your signup pipeline or existing workflows without worrying about unused quotas.

How This Compares to Competitors

Some tools offer free tiers but tie you to short-term plans. Others charge extra for high-volume sending or limit how long you can store verified data. MailTester avoids that by letting you buy credit and use it when you need it—no pressure, no expiration. This kind of flexibility is essential when scaling campaigns or building long-term mailing lists.

  • Bulk email verification lets you process thousands of addresses with the same credit system—effortlessly.
  • No mandatory subscriptions: if you only send occasional campaigns, you only pay for what you verify.
  • Even if you're paused for months, your credits are still there—no loss of value.
  • This approach aligns with industry best practices around resource control and predictable cost modeling.

For teams that rely on clean, deliverable lists, predictable costs matter. A system where credits expire can lead to wasted effort and budgeting headaches. Spamhaus and RFC 5322 both emphasize the importance of maintaining sender reputation—your list hygiene affects deliverability. With MailTester, you keep control, avoid bloat, and build reputation over time without financial friction.

Better yet, you can test the process before committing. Start with 100 free verifications to see how the API performs with your data.

Integrating MailTester into Your Email Stack: SendGrid, Mailchimp, HubSpot, Klaviyo

You can plug MailTester directly into SendGrid, Mailchimp, HubSpot, and Klaviyo to verify emails before sending—cutting bounces, improving deliverability, and protecting sender reputation. It works as a pre-send filter, validating addresses in real time or in bulk, with results tied to inbox placement scores and risk signals.

Seamless integration with major ESPs

MailTester integrates natively with the platforms you already use. Whether you're sending via SendGrid’s API, scheduling campaigns in Mailchimp, or triggering workflows in HubSpot, MailTester fits right in. You won’t need a separate dashboard or manual export. Just connect the tools, set your verification rules, and move forward.

For teams using Klaviyo, MailTester can validate subscriber lists during onboarding and flag risky or outdated addresses before campaigns launch. This reduces the chance of hitting spam traps, especially when growing lists through third-party sources.

Test before you go live—no risk

Before switching to production mode, you can test everything in a sandbox environment. This means you can run a small batch of emails through MailTester’s API and see how results appear in your ESP without touching real campaigns. It’s the safest way to confirm your setup works as expected across your stack.

Real-time feedback helps you catch edge cases early—like catch-all domains, role accounts, or temporary failures—before they hurt your sender reputation.

Once you’re in, the in-app AI assistant helps you interpret what the results mean. If an email shows as “risky” or has a low inbox placement score, the assistant suggests actions: retry, remove, or verify manually. It doesn’t guess; it explains based on the full context of the score and verdict.

And unlike some tools that just say “valid” or “invalid,” MailTester shows you why—whether it’s a typo, a temporary block, or a domain policy that rejects messages. You get not just accuracy, but clarity.

For teams serious about sender health, a good verification API should do more than filter bad addresses. It should help you understand the full delivery landscape. That’s why MailTester includes deliverability insight in every result, not just a single verdict.

See how it works in action: real-time email verification API. Start with 100 free verifications, no expiry on credits. See if it fits your stack better than alternatives like Kickbox, which don’t surface deliverability risk signals in the same depth.

What to Do When a Score is Low: A Practical Response Framework

If your email verification score is low, don’t send. A score below 60 means the address is likely invalid or high-risk. Scores 60–79 should be tested cautiously—send a single warm-up email first or source elsewhere. Scores above 80 signal readiness for bulk sends, especially with a warmed domain. Repeated low scores from a single domain may indicate sender reputation issues. Always treat deliverability scores as part of a broader health check, not a standalone signal.

Response by Score Threshold

  • Score below 60: Remove or suppress the address entirely. These are usually invalid, disposable, or blacklisted. Sending to them harms deliverability and increases bounce rates. MailTester’s bulk verification detects these early.
  • Score 60–79: Test with a single low-volume email to a few addresses. Use this to assess inbox placement. Let’s avoid mass sends until you confirm the addresses are live and engaged. Consider sourcing from verified, opt-in lists instead.
  • Score above 80: High confidence for delivery. Safe to include in mass campaigns—especially if your sending domain is already warm. Use inbox placement testing to validate real-world routing.

When Scores Are Consistently Low Across a Domain

  • Check your sender reputation: low scores across multiple domains might not be about individual addresses but your sending behavior. Tools like Spamhaus or MxToolbox help diagnose blocklist status.
  • Review your list sourcing. Are you using purchased or scraped lists? These often generate low deliverability scores and trigger spam filters.
  • Use the verification API to pre-validate new signups in real time—this stops problematic addresses before they enter your system.
  • If scores stay low after list hygiene and domain warming, consider whether you’re sending content that looks like spam—even if the domain is clean. Check your content with Return Path’s deliverability guidelines.
A deliverability score is not a verdict. It’s a signal. The best response is action, not panic.

Your score reflects the email's current state—not fate. Use it to refine your list, improve sender hygiene, and strengthen deliverability over time. With the right tools, you can turn low scores into data that improves your overall email performance.

The Bottom Line: MailTester Offers a More Complete Verification Pipeline

Kickbox functions well as a basic email validation tool, confirming syntax and domain existence. But it provides no insight into whether an email will actually land in the inbox.

Deliverability is the real test

MailTester goes beyond simple validation. It assigns a deliverability score, explains why each email is marked as valid, invalid, or risky, and tests actual inbox placement using real mail servers.

Why the upgrade matters

For teams focused on sender reputation and inbox placement, relying solely on validation tools like Kickbox is like checking a car’s engine light without testing if it drives. MailTester's full pipeline—real-time API, inbox testing, and post-verification insights—supports long-term deliverability health.

Keep reading

Ready to put this into practice? MailTester verifies emails with 98.9% accuracy — start with 100 free verifications.

Frequently asked questions

What is deliverability score in email verification?

A deliverability score estimates the chance an email will land in the inbox. It combines technical validity, domain reputation, and mailbox health signals.

Does Kickbox provide deliverability scores?

Kickbox provides basic validation but does not publish or explain its deliverability scoring model. It lacks public documentation on how scores are derived.

How accurate is MailTester's email verification API?

MailTester achieves 98.9% accuracy in independent testing across valid, invalid, catch-all, and risky address types.

Can I test real inbox delivery with Kickbox?

No. Kickbox only validates address syntax and server reachability. It does not send test emails to actual inboxes.

Is MailTester's credit system better than Kickbox's?

Yes. MailTester credits never expire, and you get 100 free verifications. Kickbox’s pricing and credit terms are less transparent.

How does MailTester handle role accounts?

It detects and flags role accounts (e.g. sales@, support@) as 'risky' based on patterns and domain behavior.

Does MailTester detect disposable email addresses?

Yes. It identifies disposable domains and marks them in the verdicts, helping avoid spam traps and low engagement.

Can I use MailTester with HubSpot or Klaviyo?

Yes. MailTester integrates directly with HubSpot, Klaviyo, Mailchimp, and SendGrid for real-time verification before send.

What’s the difference between a catch-all and a valid email?

A catch-all accepts all emails but often routes them to spam. A valid email delivers to a real mailbox, but catch-alls are technically valid without value.

Why is inbox placement testing important?

An email can be technically valid but blocked by spam filters or greylisting. Testing real inbox delivery ensures your message actually reaches the user.

How does MailTester’s AI assistant help?

It interprets verification results, suggests actions (e.g., suppress low-score addresses), and explains complex verdicts in plain English.

Does MailTester use SMTP checks?

Yes. It performs real-time SMTP checks to validate MX records, test mailbox availability, and verify server responses.