Free ZeroBounce Alternative: MailCull explained
Looking for a free ZeroBounce alternative? MailCull gives you 500 free validation credits per month with the full evidence chain on every verdict. No card.
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If you are looking for a ZeroBounce alternative, 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 (ZeroBounce ONE) you do not actually want.
MailCull is built differently. One flat $9/month price. Twenty plain-English checks per email with the SMTP reply, the MX record, and a confidence score on every verdict. And we publish our real-world accuracy quarterly, currently 91-93% on mixed lists, with the methodology fully public.
The free tier includes 500 validation credits per month, usable across Verify Email checks and Verify List uploads on the same unified pool, recurring, not one-time. Enough to clean a real test segment and feel the difference before deciding to pay anything.
The three ZeroBounce pains MailCull fixes
Pain 1: credit math
ZeroBounce sells prepaid credits. Buy a pack, watch credits tick down on every check, top up when you run out, watch unused credits expire on a schedule.
MailCull is flat. Pay $9/month for 100,000 validation credits, usable across single-email checks and list uploads on one unified pool. Unified monthly credit pool, no surprise overages, no expiration math, no charge for unknown verdicts.
Pain 2: opaque verdicts
ZeroBounce returns a status: valid, invalid, catch-all, unknown, spamtrap, abuse. They do not show you why.
MailCull returns the verdict plus a 20-code evidence chain: the SMTP reply, the resolved MX record, every step of the pipeline annotated, and a per-check 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.ioWhen 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."
Pain 3: bundle creep
ZeroBounce launched ZeroBounce ONE in September 2025: a deliverability suite that bundles validation with DMARC monitoring, blacklist monitoring, inbox placement testing, and other features. The G2 backlash was visible: customers wanting validation only ended up paying for tools they did not need.
MailCull does one thing, email validation with a verifiable evidence chain, and we do it well. We do not sell warmup, deliverability suites, finder products, or bundles. If you need DMARC monitoring, dmarcian or Postmark's free tier are excellent. If you need warmup, Mailwarm or Warmup Inbox. We are not interested in selling everything; we are interested in being the best at validation.
What "free" actually means on MailCull
ZeroBounce's "free" is 100 verifications, one-time, on signup.
MailCull's free tier is 500 validation credits every month, recurring, across single-email checks and list uploads. One credit covers one validated email, quick scan or deep scan, same cost.
Deep scan is included on Free with the full evidence chain, the same engine Pro customers run. We do not gate the moat behind paid tiers; we charge for volume (Pro is 100,000 credits/month at $9 flat) plus API + MCP access, not for visibility.
What this means in practice:
- A cold-email SDR can validate their daily 30-50 highest-stakes outbound contacts indefinitely on Free
- An agency can clean a real test segment monthly and decide based on the verdicts whether to upgrade
If you outgrow Free, Pro is $9/month flat with 100,000 validation credits, REST API access, and MCP / Connected Apps for AI agents (Claude, Cursor, and any MCP-compatible client). No per-pack credit math. Monthly only, cancel any time through Paddle.
The Microsoft 365 corporate cascade
This is where MailCull pulls ahead structurally.
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:
getuserrealm.srf: Microsoft's domain-level pre-checkGetCredentialType: primary mailbox-existence signal from Microsoft's auth endpointsAutodiscover v1: secondary fallback whenGetCredentialTypeis throttled or ambiguous- SMTP cross-check: when the cascade and SMTP disagree, we return
riskyrather than a confidently wrong verdict
The result: definitive verdicts on M365 corporate addresses where ZeroBounce gives up.
Honest accuracy comparison
ZeroBounce claims 99.6% accuracy. Independent third-party tests in 2024-2025 land between 90% and 97% on mixed lists.
MailCull publishes 91-93% real-world accuracy on mixed lists, refreshed quarterly. Methodology, dataset structure, and per-class accuracy are all public at mailcull.io/methodology:
| Class | MailCull accuracy |
|---|---|
| Consumer (Gmail, Yahoo, Outlook with real local-parts) | ~95% |
| M365 cloud-only managed tenants | ~92% |
| M365 hybrid / federated | 85-90% |
| Catch-all corporate domains | ~80% |
| Mixed bulk lists (typical customer upload) | 91-93% |
The difference between us and ZeroBounce is not accuracy, measured accuracies are similar. The difference is what we claim. We claim what we measure. They claim what their marketing wants.
When ZeroBounce is still the right answer
In the interest of honest framing: there are real cases where ZeroBounce wins:
- Your procurement gates on SOC 2 Type II today (our audit is in progress, planned month 9 of 2026)
- You are healthcare or regulated-industry and need a HIPAA BAA
- Your team already uses ZeroBounce ONE and benefits from the bundled deliverability suite
- Your buyer recognises the brand and that is what closes the procurement
If any of these are true, ZeroBounce is the right answer. We are not pretending to be the answer for every team.
Switching from ZeroBounce in 5 minutes
If you are switching:
- Sign up for MailCull Free. No credit card. 500 validation credits per month, recurring.
- Test on your real list. Take 500-1,000 addresses you have already run through ZeroBounce. Compare the verdicts.
- If you like what you see, upgrade to Pro. $9/month flat, 100,000 validation credits, REST API access, and MCP / Connected Apps for AI agents. 30-day money-back guarantee.
- We match remaining ZeroBounce credits for 30 days. Email us your last invoice and we will top up your Pro quota.
FAQ
Is MailCull truly free or is there a catch?
500 validation credits every month, recurring, usable across single-email and list workflows, one credit per validated email, quick or deep. The "catch" is the volume cap: if you need more, Pro is $9/month flat (100,000 credits + API + MCP access).
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. Documented at mailcull.io/methodology.
Why publish 91-93% accuracy when ZeroBounce claims 99%?
We claim what we measure. Independent third-party tests of ZeroBounce land at 90-97%. Our real-world accuracy is 91-93%. We would rather you trust us before sending than feel betrayed after a bounce.
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.
Is the evidence chain in the API response too?
Yes. Every API response includes the full 20-code evidence chain in the JSON, no separate endpoint required. API access is a Pro-plan feature; Free has no API access (use the dashboard checker instead).
Can I connect MailCull to Claude or Cursor?
Yes. Pro includes MCP / Connected Apps: connect any MCP-compatible AI client (Claude, Cursor, Codex, others) from Account → Connected apps. Each connection is explicit and revocable. ZeroBounce ships an MCP server too (launched December 2025), so this is a parity feature against ZeroBounce specifically. The differentiator is price + receipts: MailCull's MCP is included in the $9/mo Pro tier and every verdict the MCP returns carries the full evidence chain.
The bottom line
If you are looking for a free ZeroBounce alternative because of credit math, opaque verdicts, or bundle creep. MailCull is built for exactly those reasons.
Start free with 500 validation credits per month, no credit card. Or paste an email into the free single-email checker right now to see the evidence chain.
If you want a deeper comparison, the full MailCull vs ZeroBounce side-by-side covers pricing, accuracy data, and the feature parity table.
Start with 500 free validation credits. No credit card.
Both Free and Pro run the same scan engine — full SMTP probe, MX lookup, typo, disposable, domain checks, and the evidence chain on every verdict. The difference is the monthly credit pool (Free=500, Pro=100,000) plus Pro's API and MCP access.