If you've ever run an email list through a verification service and seen 30-40% of your emails marked as "Unknown" or "Risky," you've encountered the catch-all problem.

Most email verification providers simply give up when they hit a catch-all domain. They charge you full price, then shrug and say "we can't tell."

This guide explains exactly how catch-all verification works, why the industry's standard approach fails, and how you can actually verify these emails with certainty.

What is a Catch-All (Accept-All) Email Domain?

A catch-all domain—also known as an accept-all domain—is configured to accept emails sent to any address at that domain, regardless of whether a specific mailbox exists. The terms "catch-all" and "accept-all" are used interchangeably in the email industry and mean exactly the same thing.

Send an email to ceo@catchall-company.com, asdfghjkl@catchall-company.com, or literally-anything@catchall-company.com—the mail server accepts them all.

Why Companies Use Catch-All Configuration

Organizations enable catch-all for several legitimate reasons:

1. Preventing Email Enumeration Attacks

By accepting all emails, attackers can't determine which employee email addresses are valid. This is a security measure used by banks, government agencies, and security-conscious enterprises.

2. Capturing Misspelled Emails

If a customer sends an email to suport@company.com instead of support@company.com, a catch-all configuration ensures it still arrives.

3. Legacy Email Routing

Many enterprises use complex email routing systems where a catch-all server acts as the first point of contact, then routes emails internally.

4. Small Business Simplicity

Some small businesses use catch-all to funnel all emails to a single inbox, avoiding the need to create individual mailboxes.

How Common Are Catch-All / Accept-All Domains?

Here's what surprises most marketers: catch-all (accept-all) domains typically represent 15-40% of any B2B email list.

That's not a typo. Up to 40% of your business email list might be unverifiable using traditional methods.

For B2C lists, the percentage is lower (usually 5-15%) but still significant when you're dealing with large volumes.

How Traditional Email Verification Works

Before diving into the catch-all problem, let's understand how standard email verification works.

The SMTP Handshake Process

Email verification tools perform an SMTP handshake with the recipient's mail server. Here's the simplified process:

1. Connect to the mail server (MX record lookup)
2. Introduce ourselves: HELO verification-server.com
3. Specify sender: MAIL FROM: <verify@our-domain.com>
4. Ask about recipient: RCPT TO: <target@their-domain.com>
5. Server responds with acceptance or rejection

For most domains, this works perfectly:

250 OK → The mailbox exists, email is valid
550 User unknown → The mailbox doesn't exist, email is invalid
450 Try again later → Temporary issue (greylist), retry needed

Where It Breaks Down

Catch-all domains respond with 250 OK to every recipient address. The server accepts the email regardless of whether the actual mailbox exists.

From the verification tool's perspective, both of these look identical:

real.employee@catchall-company.com → 250 OK
gibberish12345@catchall-company.com → 250 OK

There's no way to distinguish valid from invalid using SMTP alone.

The Heuristics Approach (And Why It Fails)

Faced with the catch-all problem, most verification providers turn to heuristics—educated guesses based on patterns.

Common Heuristic Methods

Pattern Matching

Some tools check if the email follows common patterns:

firstname.lastname@domain.com → Probably valid
f.lastname@domain.com → Possibly valid
xq7mk2@domain.com → Probably invalid

The problem? Plenty of valid emails don't follow patterns (sales@, info@, hello@), and plenty of invalid emails do follow patterns.

Historical Data

Some providers maintain databases of previously verified emails. If they've seen john.smith@company.com deliver successfully before, they mark it as valid.

The problem? Email addresses get deactivated. Employees leave companies. That "historically valid" email might bounce today.

Domain Reputation Scoring

Tools assign catch-all domains a "reputation score" based on aggregate data. High-reputation domains get marked as "probably valid."

The problem? Domain reputation says nothing about individual addresses. A Fortune 500 company might have excellent domain reputation, but fakeperson123@fortune500.com still isn't valid.

Role-Based Detection

Identifying common role-based addresses (sales@, support@, info@) and assuming they're valid since most companies have these.

The problem? It only covers a tiny fraction of emails, and even role addresses can be misconfigured.

The Fundamental Flaw

All heuristic approaches share the same fundamental problem: they're guessing.

A 70% confidence score still means 30% of your "verified" emails might bounce. When you're sending 100,000 emails, that's 30,000 potential bounces destroying your sender reputation.

What "Unknown" Really Means

When you see "Unknown" or "Risky" in your verification results, here's what actually happened:

The tool detected a catch-all domain
It couldn't verify the address via SMTP
Heuristics were inconclusive
So it gave up and charged you anyway

You paid for verification. You got uncertainty.

True Verification: How to Actually Verify Catch-All Emails

There's only one way to know with certainty whether a catch-all email address is valid: send an email and see what happens.

This is exactly what True-Send verification does.

How True-Send Works

Instead of guessing, True-Send performs actual email delivery:

Send a verification email to the catch-all address
Monitor the response from the receiving server
Track bounces using real bounce handling
Confirm delivery or flag as invalid based on actual results

If the email bounces with a "user unknown" message → Invalid If the email delivers successfully → Valid

No guessing. No heuristics. No "unknown" cop-outs.

The Technical Process

Here's what happens behind the scenes:

Step 1: Initial SMTP Verification

We first run standard SMTP verification to filter out obvious invalids—syntax errors, non-existent domains, and servers that reject unknown addresses.

Step 2: Catch-All Detection

We identify domains responding with catch-all behavior by testing a known-invalid address pattern.

Step 3: True-Send Delivery

For catch-all addresses, we send a carefully crafted verification email from a dedicated sending infrastructure.

Step 4: Bounce Processing

We monitor for bounce responses over a defined window. Hard bounces ("user unknown") indicate invalid addresses. Soft bounces trigger retries.

Step 5: Delivery Confirmation

Emails that deliver without bouncing are confirmed as valid addresses.

Why Competitors Don't Do This

If True-Send is so effective, why don't all verification providers offer it?

1. Infrastructure Costs

Sending actual emails requires maintained sending infrastructure, dedicated IPs, proper authentication (SPF, DKIM, DMARC), and ongoing deliverability management.

Heuristics require only code.

2. Time

SMTP verification takes milliseconds. True-Send takes minutes to hours (waiting for bounce responses).

Most providers optimize for speed over accuracy.

3. Complexity

Bounce processing requires sophisticated systems to parse varied bounce formats from thousands of different mail servers.

It's easier to just mark emails as "unknown."

4. Reputation Risk

Sending verification emails at scale requires careful reputation management. One mistake can burn your sending IPs.

Most providers aren't willing to take that risk.

The True Cost of Bad Catch-All Data

"Risky" and "Unknown" labels seem harmless until you understand the consequences.

Sender Reputation Damage

Email providers like Gmail, Microsoft, and Yahoo track your bounce rates. When you send to invalid addresses:

Bounce rates above 2% trigger spam filters
Bounce rates above 5% can get you blacklisted
Recovery from blacklisting takes weeks or months

Each invalid catch-all email you send is a reputation hit.

Lost Revenue

If 30% of your B2B leads are marked "unknown," you have three bad options:

Don't email them → Lose 30% of potential revenue
Email them anyway → Risk your sender reputation
Use heuristics → Some will bounce, damaging reputation unpredictably

None of these are acceptable for serious email marketers.

Wasted Spend

Most verification providers charge the same rate for catch-all emails as regular verification, even though they're just returning "unknown."

You're paying full price for a non-answer.

Comparing Catch-All Verification Methods

Let's compare the different approaches objectively:

| Method | Accuracy | Speed | Cost | |--------|----------|-------|------| | SMTP Only | 0% (all "unknown") | Instant | Low | | Pattern Heuristics | 60-70% | Instant | Medium | | Historical Data | 65-75% | Instant | Medium | | Combined Heuristics | 70-80% | Instant | High | | True-Send | 99%+ | Minutes-Hours | Low |

The accuracy difference is substantial. An 80% accurate method still means 1 in 5 emails might bounce—unacceptable for professional email marketing.

Why Unlimited Verifier is Different

We built Unlimited Verifier because we were frustrated with the industry's approach to catch-all verification.

Unlimited Standard Verification

Every plan includes unlimited standard SMTP verification. Verify 1 million emails or 10 million—same price. This handles the 60-85% of emails that aren't catch-all.

Deep Verification for Edge Cases

Our Deep Verification uses advanced SMTP techniques to handle greylistintg, temporary failures, and stubborn mail servers that standard verification can't crack.

True-Send for Catch-All

When we detect catch-all domains, True-Send kicks in. Actual email delivery. Actual bounce tracking. Actual results.

The Pricing Difference

Here's where it gets interesting.

Most competitors charge $0.003-0.008 per email for standard verification, then charge premium rates for catch-all "verification" (which is often just heuristics).

Our pricing:

Unlimited standard verification included in all plans ($29-249/mo)
Deep Verification credits for advanced cases
True-Send credits for catch-all verification

At scale, you're paying a fraction of what competitors charge, and getting actual verification instead of guesses.

The 10x Guarantee

We're so confident in True-Send that we offer a 10x guarantee:

If you send to an email we marked as "Valid" and it hard bounces, send us proof. We'll refund 10x the credit cost.

No other verification provider offers this because they can't—heuristics don't provide that level of confidence.

When to Use Each Verification Type

Not every email needs True-Send verification. Here's how to optimize:

Use Standard Verification For:

Initial list cleaning
Obvious syntax and domain errors
Non-catch-all domains
High-volume, lower-value campaigns

Use Deep Verification For:

Emails that failed standard verification temporarily
Greylisted domains
Servers with strict rate limiting

Use True-Send For:

High-value B2B leads
Emails marked as catch-all/unknown
Cold outreach campaigns where reputation is critical
Any situation where you need certainty

Implementing Catch-All Verification: Best Practices

Here's how to handle catch-all emails effectively:

1. Run Standard Verification First

Always start with standard SMTP verification. This filters out:

Invalid syntax
Non-existent domains
Hard bounces from non-catch-all servers
Role-based addresses (if you want to exclude them)

This handles 60-85% of your list for free (with unlimited verification).

2. Identify Catch-All Percentage

Check how many emails are marked as catch-all. If it's:

Under 10% → Might be acceptable to skip True-Send
10-30% → True-Send recommended for important campaigns
Over 30% → True-Send essential

3. Prioritize by Value

Not all leads are equal. Use True-Send for:

Enterprise prospects
High-ticket sales opportunities
Contacts you've invested in acquiring
Campaign-critical sends

4. Monitor Results

Track your actual bounce rates after True-Send verification. You should see:

Catch-all bounce rates under 1%
Overall bounce rates well under 2%
No reputation impacts

Frequently Asked Questions

Does True-Send trigger spam filters?

No. Our verification emails are carefully crafted to appear as legitimate transactional emails. They use proper authentication, come from reputable sending infrastructure, and follow best practices.

How long does True-Send take?

Most results return within 15-30 minutes. Some servers with aggressive greylisting may take up to a few hours. We provide real-time status updates.

Can recipients see the verification email?

The verification email is designed to be unobtrusive. It's a simple, brief message that won't confuse recipients. Most mail servers process and potentially reject it before it ever hits an inbox.

What about GDPR/privacy concerns?

Sending a verification email is a legitimate interest for data quality purposes. The email contains no tracking pixels and collects no personal data beyond delivery status.

Do you verify all catch-all emails automatically?

No. True-Send uses credits because it requires actual sending infrastructure. Standard verification (unlimited) handles non-catch-all emails. You choose when to use True-Send based on your priorities.

Start Verifying Catch-All Emails Today

Stop paying for "unknown" results. Stop guessing with heuristics. Start getting real verification.

Unlimited Verifier offers:

Unlimited standard verification on all plans
True-Send catch-all verification at the lowest price in the industry
10x guarantee on all verified emails

The verification industry has been getting away with charging full price for non-answers. We're changing that.

Try Unlimited Verifier free and see the difference real catch-all verification makes.