🤔 What Is Greylisting?
Greylisting is an anti-spam technique used by mail servers that temporarily reject incoming messages from unknown senders.
- • On first contact, the server returns a temporary error (like 451 4.7.1 Try again later)
- • Legitimate senders usually retry after a short delay
- • Spammers often don't retry—so the message gets blocked
TL;DR: If your sending server doesn't retry properly, you won't land in inboxes—even with clean lists.
⚠️ Why Greylisting Matters for Cold Email
| Impact | Explanation |
|---|---|
| ⏳ Delayed delivery | Can cause several minutes (or hours) of delay if retry isn't well-configured |
| ❌ Silent failures | You may think the email was sent, but it was actually never accepted |
| 📉 Bounce classification | Bad verifiers may mark a greylisted address as invalid (false negative) |
| 🔥 Domain warming killers | In early-stage cold outreach, greylisting ruins ramp-up sequences |
💡 Common Signs of Greylisting
• Delivery logs show a temporary SMTP 4xx error
• Email eventually delivers after retry, but with a delay
• Bounce reports mention "retry later" or "greylisted"
💡 Pro tip: These signs often go unnoticed until you experience poor deliverability despite having "clean" email lists.
🛠️ How Lero Handles Greylisting (vs. Basic Verifiers)
Most cheap email verifiers treat any temporary error as a failed check. Lero doesn't.
| Feature | Cheap Verifiers | Lero |
|---|---|---|
| Retry detection | ❌ | ✅ Waits + retries |
| SMTP response interpretation | ❌ Generic fail | ✅ Greylisting-specific logic |
| Classification accuracy | ⚠️ High false negatives | ✅ Accurate "Retry-OK" state |
| Retry delay simulation | ❌ None | ✅ Simulates proper behavior |
By understanding greylisting signals, Lero avoids false bounces and gives you a true deliverability picture.
🧬 Technical Deep Dive (Optional)
Greylisting is typically implemented with a triplet memory:
- • IP of sender
- • Envelope sender (MAIL FROM)
- • Recipient address (RCPT TO)
If a combination is new, the server returns a 4xx and expects a retry within a set window. After that, the address is "whitelisted" for some time.
Developer note: This is why single-shot SMTP verification often fails with greylisted servers—they need time and a retry to respond accurately.
✅ What to Do If You're Affected
• Ensure retry logic is enabled in your mail server or ESP
• Avoid sending too many new contacts in one burst (warm-up gradually)
• Monitor SMTP responses in your delivery logs
✅ Do This
- • Set retry intervals (15 min, 1 hr, 4 hrs)
- • Use reputable ESPs with good retry logic
- • Monitor 4xx response patterns
❌ Avoid This
- • Giving up after first 4xx error
- • Marking greylisted emails as invalid
- • Blast sending without warmup
🔍 How to Test for Greylisting
Send test emails with tools like:
Example SMTP Greylisting Response:
451 4.7.1 Greylisting in action, please come back later 450 4.7.1 Try again later 421 4.2.1 Service not available - temporary delay
These are temporary errors (4xx) that should trigger a retry, not a permanent failure.
📦 TL;DR
| Concept | What It Means | Why It Matters |
|---|---|---|
| Greylisting | Temporary 4xx rejection of first email | Can delay or block cold emails |
| Good behavior | ESP retries later and succeeds | Needed for inboxing |
| Lero's edge | Detects & handles greylisting cleanly | Avoids false invalid flags |
🔗 Related Reads
❓ Frequently Asked Questions
Q1: How long does greylisting typically last?
Usually 15 minutes to 24 hours, depending on the server configuration. Most servers allow retry after 15-30 minutes.
Q2: Does greylisting affect my sender reputation?
No, greylisting is a normal part of email delivery. Proper retry behavior actually improves your reputation.
Q3: Can I bypass greylisting?
Not directly, but you can minimize its impact by warming up your domain and using reputable sending infrastructure.
Q4: How does Lero detect greylisting vs. real bounces?
Lero analyzes SMTP response codes, timing patterns, and retry behavior to distinguish temporary greylisting from permanent failures.
Q5: Which email providers use greylisting most?
Many corporate email servers and smaller ISPs use greylisting. Large providers like Gmail typically use other anti-spam methods.
🎯 Don't Let Greylisting Kill Your Cold Outreach
Lero's advanced SMTP detection accurately handles greylisting, giving you reliable validation results for your cold email campaigns.