MailCull vs ZeroBounce: side-by-side, no spin

If you are evaluating ZeroBounce against MailCull, you are probably hitting one of three things: credit math you did not sign up for, "valid" verdicts that bounced when sent, or a deliverability suite you do not actually want.

This is the side-by-side at every volume — pricing, accuracy from third-party tests, the feature parity table, and the one thing each tool is genuinely better at. No marketing spin. No fake comparisons that pretend we win on every row.

TL;DR

Choose ZeroBounce if you need SOC 2 Type II compliance for procurement today, or your team has standardized on the ZeroBounce ONE deliverability suite (DMARC monitoring, blacklist monitoring, inbox placement testing).

Choose MailCull if you want the SMTP reply and confidence score for every verdict so you can defend the decision to a client, you do not want credit math, or you want a flat $19/month price with money-back if your real-world bounce rate exceeds 3%.

Pricing at common volumes (2026)

ZeroBounce uses prepaid credits that expire and bills differently for "unknowns" depending on the credit pack. MailCull is flat monthly, no credits, and unknown verdicts do not decrement quota.

Accuracy comparison — the third-party data

This is the section nobody else writes honestly.

ZeroBounce claims 99.6% guaranteed accuracy in their marketing. Independent third-party tests in 2024-2025 land between 90% and 97% on mixed lists. The gap between claim and reality is the entire reason the ZeroBounce ONE bundle launch in September 2025 triggered visible G2 backlash about "bloated bundles."

MailCull publishes 91-93% real-world accuracy on mixed lists, refreshed quarterly with the dataset and methodology shown publicly at mailcull.io/methodology. For specific provider classes:

We publish this honestly because we would rather you trust us before sending than feel betrayed after a bounce. ZeroBounce, NeverBounce, Kickbox, and Bouncer all claim accuracy numbers their third-party tests do not support.

The evidence chain — the moat

This is the structural difference that does not show up in a feature-parity table cleanly, so it deserves its own section.

ZeroBounce returns one of about a dozen status strings per check: valid, invalid, catch-all, unknown, spamtrap, abuse. They do not show you why.

MailCull returns the verdict plus the evidence chain — twenty plain-English checks, the SMTP reply for every probe, the MX record we resolved against, and a confidence score from 0 to 1. A real example for a corporate Microsoft 365 address:

[email protected]                 ✓ deliverable
─────────────────────────────────────────────────────────
syntax_valid              Email format passed RFC 5321 validation
mx_found                  MX record: example-corp-com.mail.protection.outlook.com
provider_microsoft        Microsoft 365 protected tenant
m365_http_enum            Microsoft confirmed mailbox via GetCredentialType
m365_smtp_corroborated    SMTP probe agreed: 250 OK on RCPT TO
tls_upgraded              STARTTLS handshake completed
smtp_confirmed            Server returned 250 2.1.5 OK

Confidence: 0.94          Source IP: probe-1.mailcull.io

When a client asks "why did this address bounce?" or "why did you mark this risky?" you have the receipt. ZeroBounce returns a verdict; you cannot defend it past "the API said so."

This matters most for cold-email operators who get blamed for bounces by clients and for agencies running list hygiene as a billable service.

The Microsoft 365 cascade — where most validators fail

Microsoft 365 protected tenants (any domain whose MX hosts end in .mail.protection.outlook.com) are the hardest validation case in the category. Microsoft Exchange Online Protection (EOP) returns 250 OK for some non-existent mailboxes (anti-abuse heuristics) and 550 5.4.1 for some real mailboxes (rate-limiting probe IPs). A naive SMTP probe gets the wrong answer in both directions.

ZeroBounce, NeverBounce, and most other validators handle this by returning unknown or policy_block for M365 corporate domains — about 30-40% of typical B2B lists. Your customer pays for credits and gets back inconclusive verdicts.

MailCull runs a multi-layer cascade:

1. getuserrealm.srf — Microsoft's domain-level pre-check
2. GetCredentialType — primary mailbox-existence signal from Microsoft's auth endpoints
3. Autodiscover v1 — secondary fallback when GetCredentialType is throttled or ambiguous
4. SMTP cross-check — when the cascade and SMTP disagree, we return risky rather than a confidently wrong verdict

The result: definitive verdicts on M365 corporate addresses where most competitors give up. Trade-off: about 1.5-2 seconds per corporate validation. Worth it.

Feature parity — brutally honest

Where ZeroBounce wins genuinely: enterprise compliance certifications, the ZeroBounce ONE bundle if you actually want it, the email finder feature, and brand recognition with non-technical procurement teams.

Where MailCull wins genuinely: explainability, honest accuracy reporting, the M365 cascade, flat predictable pricing, the founder being one email away, and not paying for features you do not use.

Best for: who should pick which

ZeroBounce is the right choice for:

• Enterprise teams whose procurement requires SOC 2 Type II today
• Healthcare or regulated-industry teams needing a HIPAA BAA
• Teams already using ZeroBounce ONE and benefiting from the bundled deliverability suite
• Teams with non-technical buyers who recognise the brand

MailCull is the right choice for:

• Cold-email operators and agencies who get blamed for bounces and need defensible audit trails
• Developers integrating validation into apps who want the evidence chain in JSON
• Teams running B2B lists with significant Microsoft 365 corporate volume
• Anyone tired of credit math who wants flat monthly pricing
• Anyone who values an honest 91-93% they can plan around over a 99% claim that does not hold

Switching from ZeroBounce

If you are switching, two things help:

We match remaining ZeroBounce credits for 30 days. Email us your last invoice, and we will set your Pro account up with the equivalent volume on top of the standard monthly limit for one month.

Eat-your-own-dog-food test. Take 1,000 addresses you have already validated through ZeroBounce. Re-run them through MailCull's free tier (100 deep-scan checks per month, plus 1,000 deep-scan list rows per month — enough for this exact test). Compare the two verdict sets. If the disagreements are interesting, we will walk through them with you on a call.

FAQ

Is MailCull cheaper than ZeroBounce at every volume?

At 5K-100K addresses per month, yes — significantly. At 1M+, you should talk to us about an annual contract; we are not the right answer for every enterprise customer.

Why is MailCull's accuracy "lower" than ZeroBounce's?

It is not actually lower against real send results. ZeroBounce's headline claim is 99.6%; their measured accuracy in third-party tests is 90-97%. Our claim is 91-93%; our measured accuracy is 91-93%. We claim what we measure.

Does MailCull do SMTP mailbox verification?

Yes — full SMTP probing with STARTTLS opportunistic upgrade, FCrDNS-matched EHLO, IPv6 dual-stack support, and a per-MX rate limiter. Plus the M365 HTTP enumeration cascade for corporate Microsoft 365 tenants. The full pipeline is documented at mailcull.io/methodology.

What about catch-all domains?

Catch-all domains accept every recipient at the SMTP layer, so a single probe always returns 250. MailCull runs a multi-probe consensus with random local-parts to detect catch-all behaviour and marks those addresses risky with a confidence score, never deliverable. ZeroBounce flags catch-alls but does not always probe consensus.

Can I get a refund?

Yes — 30-day money-back, no questions asked. If your real-world bounce rate against the list we said was deliverable exceeds 3%, full refund. Send us the bounce report from your ESP.

What if I need both — MailCull for validation and ZeroBounce ONE for DMARC monitoring?

Run both. They do not conflict. Use MailCull for the validation step and ZeroBounce or a dedicated tool (dmarcian, Postmark, Valimail) for deliverability monitoring. We do not bundle on purpose.

The bottom line

ZeroBounce is the brand-name premium incumbent. MailCull is the explainable indie alternative built for senders who need receipts, not just verdicts.

If you want to test the difference, start with the free tier — 100 deep-scan single-checks per month, full evidence chain on every one, no credit card.

If you are ready to switch, Pro is $19/month flat with 5,000 single checks plus 100,000 list rows monthly. Money-back in 30 days. Cancel through Paddle anytime.

Or paste any email address into the free single-email checker right now to see the evidence chain for yourself.