Why Email Verification Fails: Troubleshooting Common Issues

Email verification should be simple. You send an email address to an API, it tells you if it's valid, and you move on with your life.

Except it doesn't work that way.

Valid emails get rejected. Invalid emails pass verification. Catch-all domains return "unknown" for everything. Some mail servers take forever to respond. Others reject your verification attempts entirely. And you're paying for checks that don't actually verify anything.

I've spent years helping companies debug their verification implementations. The same problems come up over and over. Let me walk you through exactly why verification fails and how to fix each issue.

The Catch-All Domain Problem

Catch-all domains are the bane of email verification. These are mail servers configured to accept mail for ANY address at their domain, whether or not the mailbox actually exists.

When you verify "randomjunk@catchalldomain.com", the mail server says "sure, I'll accept that." But if you actually send an email, it might bounce because the mailbox doesn't exist. Or it might go to a general inbox that nobody monitors.

This happens more often than you'd think. Many companies configure their domains as catch-all to avoid missing emails due to typos. If someone emails "saels@company.com" instead of "sales@company.com", the catch-all setup ensures they still receive the message.

Here's the technical issue: during SMTP verification, the verification service connects to the mail server and asks "do you accept mail for this address?" A catch-all server responds "yes" to EVERY address, even non-existent ones.

How to Detect Catch-All Domains

Good verification services detect catch-all domains by testing multiple random addresses. If the server accepts "sdfkljsdflkj@domain.com" and "qwertyuiop@domain.com" and "asdfghjkl@domain.com", it's almost certainly a catch-all.

The BulkEmailChecker API automatically detects catch-all domains and returns detailed information in the response, allowing you to handle these cases appropriately in your application logic.

Greylisting and Temporary Rejections

Greylisting is an anti-spam technique where mail servers temporarily reject messages from unknown senders. The server says "try again later" and expects legitimate senders to retry in a few minutes.

For email verification, this creates a problem. When you attempt SMTP verification, the greylisting server rejects your connection with a temporary error code. The verification service can't determine if the mailbox exists because the server won't even check.

Greylisting affects about 10-15% of email verifications. It's particularly common with smaller businesses using hosted email solutions.

Timeout and Connection Issues

Some mail servers are just slow. They take 30, 60, or even 90 seconds to respond to SMTP verification attempts. Others don't respond at all - they accept the connection but never send a response.

This creates timeout issues. Your verification service waits for a response, doesn't get one, and eventually gives up.

The right timeout setting depends on your use case. For real-time form validation where users are waiting, you need fast results. For bulk list cleaning where speed doesn't matter, use longer timeouts for maximum accuracy.

False Positives: Valid Emails Marked Invalid

Nothing is more frustrating than rejecting a valid email address. Someone tries to sign up for your service, enters their legitimate email, and your form tells them it's invalid.

False positives happen for several reasons.

Overly Strict Syntax Validation

Some verification systems reject emails that don't fit their expected format, even though the addresses are technically valid according to RFC standards.

For example, these are all valid email addresses that some systems incorrectly reject:

• "user+tag@example.com" - Plus addressing (very common)
• "user.name@example.com" - Dots in the local part
• "user_name@example.com" - Underscores
• "user-name@example.com" - Hyphens

Choosing the Right Verification Method

Not all verification is equal. Understanding the different methods helps you choose the right approach.

Bulk Email Checker uses full SMTP verification for all checks, achieving 99.7% accuracy. You can test it with the free email verification tool before integrating into your workflow.

Common Implementation Mistakes

Let me save you from the mistakes I see constantly.

Treating "Unknown" as "Invalid"

The biggest mistake: rejecting emails that return "unknown" status. Unknown means "couldn't verify" not "definitely invalid."

Many valid emails will return unknown due to catch-all, greylisting, or temporary issues. Rejecting them all means losing legitimate leads.

Instead: Accept unknown emails but flag them. Lower their lead score. Route them differently. But don't reject them outright.

Using Verification Services Without SMTP Checks

Many cheap verification services only check syntax and DNS. They're fast and cheap because they don't do real verification.

These services catch maybe 30% of invalid emails. The other 70% slip through and bounce when you send them messages.

Use services that perform full SMTP verification. Yes, it's slower and costs more, but it actually works. Check out BulkEmailChecker's pay-as-you-go pricing for cost-effective SMTP verification at scale.

Frequently Asked Questions

Why does verification say an email is invalid when I know it works?

Several reasons: The mail server might be temporarily down. It might be greylisting and rejecting verification attempts. Or it could be a catch-all domain that the verification service correctly marked as unknown. Check the specific error code in the response - it'll tell you exactly why.

Can email verification be 100% accurate?

No. Catch-all domains, greylisting, email forwarding, and full mailboxes all create scenarios where verification can't give definitive answers. The best services achieve 99%+ accuracy for clear pass/fail cases, but about 10-15% of emails will return "unknown" status where verification is inconclusive.

How often should I re-verify email addresses?

For active contacts, re-verify every 90 days. Email addresses become invalid at a rate of about 22% per year, so quarterly verification catches most changes. For critical contacts like customers, verify monthly. For cold lists, verify before each major campaign.

What should I do with emails that return "unknown" status?

Accept them but flag them in your system. Lower their lead score. Consider sending a confirmation email to verify a real person monitors the address. Don't reject unknown emails outright - many are legitimate addresses that just couldn't be verified due to technical reasons.

Conclusion

Email verification fails for predictable, solvable reasons. Catch-all domains, greylisting, timeouts, rate limiting - every issue has a workaround.

The key is using a verification service that handles these problems automatically. You shouldn't need to know about greylisting retry logic or per-domain throttling. The API should handle it behind the scenes.

When verification fails, check the error codes. They tell you exactly what went wrong and how to fix it. Don't just accept "it's not working" - dig into the specific failures and address them systematically.

Start by testing with the free email verification tool to see how different email types behave, then integrate the full API when you're ready to automate.