# How to Check if an Email Is Catch-All (Accept-All)

URL: https://leadraft.co/blog/how-to-check-if-email-is-catch-all
Summary: To check if an email is catch-all, ask the mail server about an address that can't exist. How the test works, why a verifier should run it, and which tools do.

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# How to check if an email domain is catch-all

To check if an email is catch-all, ask the mail server about an address that can't exist. How the test works, why a verifier should run it, and which tools do.

The Leadraft teamLeadraft

Published

November 3, 2026

Read time

5 min

Category

Deliverability

To check if an email domain is catch-all, ask its mail server whether it would accept mail for an address that can't possibly exist, such as a long random string at the same domain. If the server says yes to that address too, it accepts everything, and the domain is catch-all.

You can run that test by hand over SMTP, but in practice it's better to let an email verifier do it, because the test only gives honest answers when it runs from a mail-friendly server with a clean reputation.

## How does the catch-all test work?[#](#how-does-the-catch-all-test-work)

The test compares two answers from the same mail server. A checker opens an SMTP conversation, the same one a real delivery uses, and names two recipients in turn with the `RCPT TO` command: the address you care about, then a made-up one.

On a normal server, real and made-up addresses get different answers. A catch-all server says 250 OK to both, which is why those addresses get an Accept-all grade instead of Verified.

Scroll horizontally if needed →

| Real address | Made-up address | What it means |
| --- | --- | --- |
| `250` accepted | `550` rejected | Normal server. The real mailbox exists |
| `550` rejected | `550` rejected | Normal server. The real address doesn't exist |
| `250` accepted | `250` accepted | **Catch-all.** The server accepts everything, so the first yes proves nothing |
| `4xx` or timeout | any | No answer yet. Retry later; don't conclude anything |

The checker hangs up before the `DATA` step, so no message is sent. The full step-by-step pipeline, with reply codes, is in [how email verification works](https://leadraft.co/blog/how-email-verification-works).

## How do I check for catch-all manually?[#](#how-do-i-check-for-catch-all-manually)

Conceptually, a manual check is five steps:

1.  **Look up the domain's MX records** to find its mail server. Our free [email domain checker](https://leadraft.co/tools/email-domain-checker) shows them.
2.  **Connect to that server on port 25** and introduce yourself (`EHLO`).
3.  **Name a sender** with `MAIL FROM`.
4.  **Name a made-up recipient** with `RCPT TO`, something like a 20-character random string at the domain.
5.  **Read the reply.** A `250` means the server accepts unknown addresses: catch-all. A `550`\-type reply means it rejects them, so its answer for a real address means something.

Note that the email domain checker covers step 1 only. It reads DNS (MX, SPF and DMARC records) and never talks to the mail server, so it can't tell you whether a domain is catch-all.

## Why shouldn't I run SMTP checks from my own computer?[#](#why-shouldnt-i-run-smtp-checks-from-my-own-computer)

Because where the check comes from changes the answer, and can harm your network's reputation.

-   **Port 25 is often blocked.** Many internet providers block outbound port 25 from home connections. Cloud platforms do too: Amazon EC2 blocks port 25 to public addresses by default, and Google Compute Engine blocks connections to destination port 25 outside your network.
-   **Home IPs are on blocklists.** Spamhaus's Policy Blocklist lists end-user IP ranges "from which email should never be sent directly to the final destination." Many mail servers refuse or distrust connections from those ranges, so you get errors, not answers.
-   **Probing looks like harvesting.** Checking many addresses at one domain from one IP looks like someone mining the domain for valid mailboxes. Microsoft's Outlook.com sender policy names "namespace mining" as a prohibited practice. Servers that notice may block the IP, and the block can hit your real email if it shares that IP.
-   **Greylisting gives false negatives.** Some servers answer unknown senders with a temporary `4xx` code on purpose. A one-off manual check reads that as a failure.

A one-off check of a single domain is unlikely to cause trouble. Running it over a list is what a verifier is for.

## Which tools check whether an email is catch-all?[#](#which-tools-check-whether-an-email-is-catch-all)

Most dedicated email verifiers run the catch-all test as part of every check and label the result. The label differs by tool: "catch-all", "accept-all", "accept all" or "risky". Examples include ZeroBounce, NeverBounce, MillionVerifier, Bouncer and Hunter's email verifier. Most offer a small number of free checks; see each vendor's pricing page for current limits.

Scroll horizontally if needed →

| Tool type | Detects catch-all? | Notes |
| --- | --- | --- |
| DNS or MX checker | No | Shows whether the domain takes mail at all |
| Email verifier (SMTP) | Yes | Runs the made-up-address test |
| Lead tool with built-in verification | Depends | Ask whether it checks the mailbox over SMTP or only the domain |

The last row matters. Some tools that call their check "verification" only look up the domain. We explain the difference in [domain check vs mailbox check](https://leadraft.co/blog/domain-check-vs-mailbox-check). A quick test: if a tool never returns a catch-all result, it probably isn't asking the mail server.

## What should I do with catch-all addresses?[#](#what-should-i-do-with-catch-all-addresses)

Treat them as unconfirmed and send them separately. Hunter tested this: in its sample, 38% of email domains were accept-all, and a test send to 100 accept-all addresses bounced 27% of the time, against 1% for 100 verified addresses. Send verified addresses first, send catch-alls later in small batches, and prefer addresses the business published on its own site over guessed ones. Our guide to [catch-all emails](https://leadraft.co/blog/catch-all-emails-explained) has the full decision rules.

## FAQ[#](#faq)

### How do I know if an email domain is catch-all?[#](#how-do-i-know-if-an-email-domain-is-catch-all)

Ask the domain's mail server, during an SMTP conversation, whether it would accept mail for a random address that can't exist. If it accepts that address too, the domain is catch-all. Email verifiers run this test for you and report the result as catch-all, accept-all or risky.

### Can I check for catch-all without sending an email?[#](#can-i-check-for-catch-all-without-sending-an-email)

Yes. The test stops before any message is sent: the checker names the recipient with `RCPT TO`, reads the server's reply, and disconnects. No email is delivered to anyone.

### Does an MX lookup or domain checker show if a domain is catch-all?[#](#does-an-mx-lookup-or-domain-checker-show-if-a-domain-is-catch-all)

No. DNS records such as MX, SPF and DMARC show whether a domain can receive mail and how it's configured, but whether the server accepts every address is only visible by asking the server itself over SMTP.

### Should I run catch-all checks from my own computer?[#](#should-i-run-catch-all-checks-from-my-own-computer)

Usually not. Many home and office internet connections block outbound port 25, home IP ranges are often listed on blocklists such as Spamhaus's Policy Blocklist, and repeated probes from one IP can look like address harvesting. Use a verifier that runs the checks from servers set up for it.

### Is a catch-all email address invalid?[#](#is-a-catch-all-email-address-invalid)

No. It means the check couldn't tell. The mailbox might exist and be read every day, or mail might go to an unread inbox or bounce later. Treat it as unconfirmed: a maybe, never a yes.

## Sources[#](#sources)

1.  IETF, [RFC 5321: Simple Mail Transfer Protocol](https://www.rfc-editor.org/rfc/rfc5321) (`RCPT TO`, reply codes).
2.  IETF, [RFC 6647: Email Greylisting](https://www.rfc-editor.org/rfc/rfc6647).
3.  Spamhaus, [Policy Blocklist (PBL)](https://www.spamhaus.org/blocklists/policy-blocklist/).
4.  Amazon Web Services, [Amazon EC2 service quotas](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-resource-limits.html) (port 25 restricted by default).
5.  Google Cloud, [Sending email from an instance](https://docs.cloud.google.com/compute/docs/tutorials/sending-mail) (destination port 25 blocked outside the VPC network).
6.  Microsoft, [Outlook.com Postmaster: Policies, Practices, and Guidelines](https://substrate.office.com/ip-domain-management-snds/postmaster/Policies) (namespace mining).
7.  Hunter, [Accept-all (catch-all) emails](https://hunter.io/email-verification-guide/accept-all-catch-all) (November 4, 2025).

## Keep reading

-   [
    
    November 3, 2026Deliverability
    
    ### Domain check vs mailbox check: what email verification actually proves
    
    An MX (domain) check proves a domain takes mail. An SMTP (mailbox) check asks about the address. What each proves, why some verifiers skip SMTP, how to tell.
    
    Read article
    
    ](https://leadraft.co/blog/domain-check-vs-mailbox-check)
-   [
    
    October 31, 2026Deliverability
    
    ### What's a good cold email bounce rate? (And how to get yours under it)
    
    Aim for a cold email bounce rate under 2%. What Gmail, Yahoo, Outlook and sending tools actually publish, what causes bounces, and how to fix a high rate.
    
    Read article
    
    ](https://leadraft.co/blog/cold-email-bounce-rate)
-   [
    
    October 31, 2026Deliverability
    
    ### How email verification works: syntax, MX, SMTP and catch-all, explained
    
    How email verifiers check an address without sending mail: syntax, MX records, the SMTP RCPT TO test, catch-all detection, their limits, and when to re-verify.
    
    Read article
    
    ](https://leadraft.co/blog/how-email-verification-works)

Last built: 2026-11-09