Email Greylisting: What It Is and Why Your Verification Results Show Unknown

You uploaded a list of 5,000 emails for verification. Most came back as "passed" or "failed" - exactly what you expected. But 400 of them? "Unknown." No clear answer. And now you're stuck trying to figure out whether those contacts are worth emailing or whether they'll bounce and damage your sender reputation.

That "unknown" status usually points to one thing: greylisting. It's an anti-spam technique that's been around for years, and it creates a specific challenge for email verification services. Understanding how it works will help you make better decisions about those ambiguous results - and choose a verification service that handles it properly.

What Is Email Greylisting?

Email greylisting is a spam filtering method where a mail server temporarily rejects messages from unrecognized senders, then accepts the message on a subsequent delivery attempt. It works because legitimate mail servers always retry delivery after a temporary rejection, while spam bots typically don't.

Think of it like a bouncer at a club who tells first-time visitors to come back in 15 minutes. Real guests will wait and come back. Spammers - who are trying to blast millions of messages as fast as possible - won't bother retrying. They move on to easier targets.

The technique was first proposed in 2003 and it's still widely used today, especially by corporate mail servers and smaller hosting providers. It's cheap to implement, requires almost zero configuration, and stops a significant amount of spam without any false positives from legitimate senders (assuming their mail server properly retries).

How Greylisting Works at the SMTP Level

Here's the technical play-by-play of what happens when a mail server uses greylisting:

Step 1: Initial connection. Your mail server connects to the recipient's server and begins the SMTP handshake. It identifies itself with a HELO/EHLO command, specifies the sender (MAIL FROM), and the recipient (RCPT TO).

Step 2: Temporary rejection. The receiving server checks three things - the sending IP, the sender address, and the recipient address. This combination is called a "triplet." If it's the first time seeing this triplet, the server responds with a 4xx status code (typically 450 or 451), meaning "try again later." This is a soft rejection, not a permanent failure.

Step 3: The wait. The greylisting server stores the triplet and sets a timer. The delay varies by implementation - anywhere from 1 minute to several hours, though 15 minutes is the most common default.

Step 4: Retry and acceptance. A properly configured mail server queues the message and retries after its own delay interval. When the retry arrives and enough time has passed, the greylisting server recognizes the triplet, accepts the message, and usually adds it to a whitelist so future messages from the same triplet skip the delay.

The key insight: spam bots almost never retry. They're optimized for volume, sending millions of messages per hour. Queuing and retrying a temporarily rejected message isn't worth their time. So greylisting stops them without needing to analyze message content, check reputation databases, or run any resource-intensive filtering.

Why Greylisting Breaks Email Verification

Here's where it gets problematic. Email verification services work by performing an SMTP handshake with the recipient's mail server - essentially asking "does this mailbox exist?" without actually sending a message. The server's response determines whether the email is valid.

When a verification service connects to a greylisted server, the server sees an unrecognized connection and responds with a 4xx temporary rejection. A basic verification tool interprets this as "I can't determine if this email exists" and marks it as "unknown."

The result? A perfectly valid email address gets flagged as unverifiable. Not because the address is bad, but because the server's anti-spam system blocked the verification attempt.

This creates a frustrating situation. You might have hundreds or thousands of genuinely valid email addresses in your list that return "unknown" purely because their mail servers use greylisting. If you throw those contacts away, you're losing real prospects. If you email them blindly, you're taking a risk on unverified data.

Greylisting vs. Catch-All Domains

People often confuse greylisting unknowns with catch-all unknowns. They're completely different problems.

Greylisting is a server behavior. The server temporarily rejects the verification attempt because it doesn't recognize the connecting IP. Given enough time and a retry, the email address can be properly verified. The address might be valid or invalid - you just can't tell yet because the door was shut before you could ask.

Catch-all domains are a configuration choice. The domain is set up to accept email sent to any address, whether the specific mailbox exists or not. Send to random-gibberish@catchalldomain.com and it won't bounce. This makes verification impossible because the server always says "yes" - even for addresses that nobody reads.

The practical difference matters for how you handle each type. Greylisted unknowns can often be resolved by using a verification service with retry logic (more on that below). Catch-all unknowns are fundamentally unresolvable through SMTP verification alone - you need to use other signals like engagement data, domain reputation, and email pattern analysis.

Bulk Email Checker differentiates between these two scenarios in its API response. A greylisted result returns an is_greylisting event, while a catch-all domain returns is_catchall. This distinction lets you build separate handling logic for each case.

What to Do with Greylisted Unknown Results

You've got a batch of unknown results. Here's a practical playbook for handling them.

First, separate greylisted from catch-all unknowns. If your verification service distinguishes between them (Bulk Email Checker does), sort them into different buckets. They need different treatment.

For greylisted unknowns:

Re-verify them. Wait 30-60 minutes, then run them through verification again. Many greylisting implementations whitelist an IP after the first successful retry, so the second attempt often goes through. With Bulk Email Checker, this retry logic is built into the verification process - the system automatically waits and retries when it detects greylisting.

If re-verification still returns unknown, tag these contacts in your email list but don't discard them. Send to them in small batches (50-100 at a time) during your next campaign and monitor bounce rates. If they deliver successfully, move them back to your main list. If they bounce, remove them.

For catch-all unknowns:

These are higher risk. Create a separate segment and limit your exposure. Send to no more than 5-10% of your catch-all contacts per campaign, watch for bounces and spam complaints, and gradually expand or prune based on results.

How Bulk Email Checker Handles Greylisting

Not all verification services handle greylisting well. Basic tools connect, get rejected, and immediately mark the address as unknown. That's leaving accuracy on the table.

Bulk Email Checker uses anti-greylisting technology built into its verification engine. Here's what happens when it encounters a greylisted server:

Detection: The system recognizes the 4xx response pattern as greylisting rather than an actual deliverability issue. This is important - not every 4xx means greylisting. Sometimes it means the server is overloaded, rate-limiting, or experiencing a temporary error. Bulk Email Checker's 17+ factor verification system can distinguish between these scenarios.

Retry with backoff: Instead of giving up, the system queues the address and retries after the typical greylisting window. For bulk verification jobs, this means your results take slightly longer to complete, but your accuracy rate goes up significantly.

Clear status reporting: When greylisting is detected, the API response includes "event": "is_greylisting" so you know exactly why a result is unknown. This transparency lets you make informed decisions about how to handle each contact.

The result? Fewer unknown results and more definitive passed/failed statuses. You get cleaner data to work with, which means fewer risky sends and better protection for your sender reputation.

For high-volume senders who can't afford ambiguity, Bulk Email Checker's unlimited API plans provide the throughput needed to verify and re-verify large lists without worrying about per-email costs. And with pay-as-you-go credits that never expire, even occasional batch cleanups are cost-effective.

Frequently Asked Questions

What percentage of email servers use greylisting?

There's no definitive count, but greylisting remains common among corporate mail servers, university systems, and smaller hosting providers. Enterprise environments using Microsoft Exchange or Google Workspace generally don't use traditional greylisting, as they have more sophisticated spam filtering. You're most likely to encounter greylisting with SMB-oriented hosting providers and self-managed mail servers.

Does greylisting affect email deliverability?

Not for legitimate senders. If your mail server is properly configured (it queues and retries after temporary rejections, as required by SMTP standards), greylisting only causes a brief delay on first contact. Once your sending IP is whitelisted, subsequent emails deliver instantly. The delay typically ranges from 5 to 30 minutes.

Can I tell if a specific domain uses greylisting?

Not easily from the outside. Greylisting is invisible to the sender until you try to deliver a message. You can sometimes infer it from delayed delivery times - if a first email to a domain takes 15-20 minutes to arrive but subsequent emails are instant, greylisting is likely in use. Using Bulk Email Checker's free email checker can help identify greylisted domains through the is_greylisting event flag.

How is greylisting different from blacklisting?

Blacklisting permanently blocks a sender. Messages from blacklisted IPs or domains are rejected outright (5xx error) and won't be delivered no matter how many times you retry. Greylisting only temporarily delays delivery. It's a one-time hurdle for new senders, not a permanent block. Think of blacklisting as a locked door and greylisting as a "please knock again in 15 minutes" sign.

Should I remove unknown results from my email list?

No - at least not immediately. Unknown results from greylisting often represent valid email addresses. Re-verify first using a service with anti-greylisting technology. If they're still unknown after retries, segment them separately and test with small sends. Only remove them if they actually bounce when you send a real message.