SMTP Error 421 4.7.29 – Suspicious Activity Detected: Fixing Email Sending Issues

What is SMTP Error 421 4.7.29?

SMTP Error 421 4.7.29 is a temporary email rejection due to suspicious actions from your sending domain/IP address. It serves as a protective measure taken by email providers to block spam and prevent phishing and abuse from reaching their users.

This error means your emails will not be delivered. Instead, it will defer the emails temporarily. This is not a hard bounce—meaning that it is not a permanent rejection (as is the case for a permanent rejection code, such as a 550 error) and you still have the opportunity to rectify the problem and resend the email. If the root cause is not remediated, email providers may continue to tighten the screws and problems with deliverability can become much larger.

How email providers use SMTP Error 421 4.7.29 to prevent spam

Gmail, Outlook, and Yahoo, to name a few, are top ESPs that closely track sending patterns to filter spam. They rely on machine learning, real-time spam filtering and authentication checks to decide whether an email should be delivered, deferred or rejected. Simply put, SMTP Error 421 4.7.29 is an early warning sign that your email practices might need to be assessed.

Common causes of SMTP Error 421 4.7.29

Understanding the root causes of this error is the first step to resolving it and preventing future issues. Here is a list of the most common reasons email services may temporarily block your emails.

1. Sudden increase in email volume

Email providers have algorithms in place to identify this type of anomaly in sending behaviour. For example, if you normally send 200 emails per day from your domain and then suddenly go to 10,000 emails in an hour, that looks suspicious. Such a spike is usually consistent with compromised accounts, spam campaigns, or bot-driven attacks leading to temporary restrictions.

2. Failed authentication checks (SPF, DKIM, DMARC)

Email authentication protocols are essential for verifying sender legitimacy. When these records are absent, incorrectly configured, or malfunctioning, your emails may generate SMTP Error 421 4.7.29.

• SPF (Sender Policy Framework): Specifies allowed email servers that can send emails using your domain. If your SPF record does not include the appropriate mail servers, emails will be unable to pass authentication.
• DKIM (DomainKeys Identified Mail): Uses cryptographic signatures to ensure the integrity of the emails received. Receiving servers may reject emails if DKIM signing is not enabled or not configured properly.
• DMARC (Domain-based Message Authentication, Reporting & Conformance): Matches the SPF and DKIM policies to reduce the risk of spoofing and phishing attacks.  If there is a misconfigured DMARC policy (e.g., p=reject without proper alignment), your email may be blocked on its way to the inbox. 

🔖 Related Reading: SPF, DKIM, and DMARC: Boosting Email Security and Deliverability

3. Spam-like behavior & user complaints

Spam complaints indicate that recipients are annoyed or suspicious of your emails. Email providers monitor these rates closely and will take action if:

• The complaint rate exceeds 0.1% (1 per 1,000 emails sent).
• A significant portion of emails land in spam folders.
• The domain has been flagged for sending unsolicited emails.

Signs that your emails are being flagged as spam

• Increase in bounce rates with “message rejected” errors.
• Lower open rates than usual.
• Emails landing in the spam folder instead of inboxes.

4. Low sender reputation & blacklist issues

Your sender reputation determines how email providers treat your messages. If your reputation is low, ISPs may temporarily defer emails with SMTP Error 421 4.7.29—or even block them entirely. Sender reputation is a score assigned to your email domain and IP address based on factors like:

• Email sending patterns (consistent vs. erratic)
• Spam complaints and bounce rates
• Engagement rates (opens, clicks, and replies)
• Blacklist status

How to fix SMTP Error 421 4.7.29?

If you’ve encountered SMTP Error 421 4.7.29, it’s a signal that your email-sending practices need attention. The good news is that this issue is temporary and can be resolved by identifying and addressing the root cause. Follow these six essential steps to restore email deliverability and prevent further disruptions.

Step 1: Identify the root cause

Before making any changes, you need to pinpoint why your emails are being flagged. Here’s how to diagnose the issue:

• Using email logs and error reports. Review SMTP bounce messages to check if multiple emails are failing due to 421 errors. Look for patterns. Are emails failing for specific domains (e.g., Gmail, Outlook)? Are they being sent at a high volume?
• Checking Google Postmaster tools and Microsoft SNDS. If you send emails to Gmail users, Postmaster Tools provides data on domain reputation, spam rates, and delivery errors. For Outlook and Microsoft email users, SNDS helps monitor your sender reputation and blocklist status.

👉 Action: Log into these tools and check for any alerts, spam complaints, or authentication failures that might be triggering the error.

Step 2: Slow down email sending 

Email providers limit senders who suddenly increase email volume, as it resembles spam behavior. If your sending rate surged too quickly, ISPs may temporarily defer your emails. The solution is to gradually increase your email volume, and here’s why:

• Email providers expect a steady, predictable sending pattern.
• Spikes in volume can trigger spam filters and reputation issues.
• Throttling helps you avoid email blocks while improving engagement rates.

Best practices for warming up a domain:

• Start slow. If your domain is new, begin with 50-100 emails per day and increase gradually over weeks.
• Use a warm-up tool like Warmy.io to automate and optimize your sending pattern.
• Engage with real recipients. Start with known contacts who are likely to open and reply.
• Monitor performance. Keep an eye on open rates, spam complaints, and bounce rates.

👉 Action: Adjust your sending schedule to ramp up email volume gradually and avoid triggering future restrictions.

🔖 Related Reading: The Science and Process of Warming Up Newly Created Email Domains

Step 3: Ensure proper email authentication

Failing authentication checks (SPF, DKIM, DMARC) is one of the most common causes of SMTP Error 421 4.7.29. Fixing these issues improves your domain reputation and email deliverability.

Step-by-step guide to setting up SPF, DKIM, and DMARC correctly

1. Check your SPF record
   
   
   • Your SPF record should include all authorized email servers.
   • Example SPF record:
     v=spf1 include:_spf.google.com include:spf.protection.outlook.com -all
   • Avoid having multiple SPF records, as it can cause failures.
     
     
2. Ensure DKIM Signing is Enabled
   
   
   • DKIM uses cryptographic signatures to verify emails.
   • Generate a DKIM key in your email provider settings and publish the TXT record in DNS.
   • Use a tool like MXToolbox to validate your DKIM signature.
3. Set Up DMARC Policy
   
   
   • A basic DMARC policy:
     v=DMARC1; p=none; rua=mailto:[email protected]
   • Start with p=none (monitor mode) before enforcing stricter policies.

👉 Action: Verify your SPF, DKIM, and DMARC settings and correct any misconfigurations.

Step 4: Clean up your email lists

Sending to invalid, unengaged, or spam-trap emails reduces your sender reputation and increases the risk of SMTP 421 errors. Hard bounces and unknown recipients harm your reputation, so it is encouraged to remove addresses that haven’t engaged with your emails in 3-6 months.

You can also set up engagement-based segmentation such as:

• High-engagement recipients (opens & replies) → Increase sending frequency.
• Low-engagement recipients (no opens in 6 months) → Re-engagement campaign or removal.
• Unsubscribed & Complaints → Remove immediately.

Step 5: Improve email content & sending practices

The way you write and structure your emails can trigger spam filters. 

DON’T use:

• “FREE!!!”, “100% GUARANTEE!”, “WIN $$$” (spammy words)
• Excessive caps, exclamation marks, or emojis in subject lines
• Too many links or images without text

DO:

• Personalize emails with recipient’s name
• Keep a healthy text-to-image ratio (at least 60% text)
• Use a consistent sender name (not generic “no-reply” emails).
• Align email content with your audience’s interests and past engagement.
• Send from a recognizable business domain (avoid free domains like @gmail.com).
• Test emails using Warmy.io’s Spam Checker

👉 Action: Test your emails before sending and follow best practices for content, design, and personalization.

Step 5: Check and request removal from blacklists

If your domain or IP address is blacklisted, email providers will restrict your emails. To know if your domain is blacklisted, you can use tools like Warmy’s free deliverability test.

How to request delisting from major blacklists

1. Identify the reason for blacklisting (high spam complaints, poor reputation).
2. Fix authentication issues (SPF, DKIM, DMARC).
3. Submit a removal request to the blacklist provider.
   • Spamhaus: Use their delisting form.
   • SURBL & UCEPROTECT: Follow instructions on their site.
4. Monitor sending behavior to avoid getting listed again.

How Warmy.io helps prevent SMTP Error 421 4.7.29 and other email deliverability issues

SMTP Error 421 4.7.29 is just one of many email deliverability challenges businesses face. Reputation issues, poor domain health, authentication failures, and spam complaints can all lead to email throttling, deferrals, or outright blocks. Warmy.io is an AI-driven email deliverability platform that ensures your emails consistently reach the inbox instead of spam or getting blocked. It provides automated warmup, reputation monitoring, and intelligent deliverability optimization to protect your sending domain and prevent SMTP 421 errors from occurring.

Let’s explore how Warmy.io actively prevents and mitigates email sending issues with the following key features:

AI-powered email warmup and advanced seed lists to build and maintain sender reputation

Many SMTP 421 errors occur because of sudden spikes in email volume, especially from new or inactive domains. Email providers flag such activity as suspicious, leading to temporary blocks. Warmy helps warmup domains the right way by:

• Gradually increases your sending volume over time, mimicking human-like interactions (opens, replies, and removals from spam).
• Using advanced seed lists to engage with real, active email addresses to improve reputation.
• Providing insights and analytics to help senders identify and resolve deliverability issues.