If your emails are suddenly bouncing back with SMTP Error 421 4.7.28, your IP address has been temporarily blocked by the recipient’s email service provider (ESP). No email sender wants this, and all email senders want to solve this.
This error typically occurs due to:
- Spam-like sending behavior (high volume, poor engagement, or suspicious patterns).
- Failed authentication checks (issues with SPF, DKIM, or DMARC).
- Reputation issues (your IP has been flagged due to past sending history).
If ignored, a temporary block becomes a permanent blacklist, which translates into long-term damage to your sender reputation.
In this guide, we’ll explain why this happens and how to find out if your IP is flagged, along with step-by-step solutions to fix it.
What is SMTP Error 421 4.7.28?
SMTP Error 421 4.7.28 is a temporary block imposed by an email service provider (ESP) when your sending IP exhibits suspicious or non-compliant behavior. A 4.7.28 subcode indicates that the recipient’s email provider has temporarily blocked your IP address, potentially due to security concerns.
While your email is not permanently rejected, this error should not be taken lightly. The receiving server has decided to pause acceptance due to potential spam risks or security concerns—and you were most likely identified as one even if you are legitimate.
If this error is not addressed and it keeps happening, temporary blocks can escalate to permanent blacklisting. This can significantly impact your ability to send emails in the long run.
Common causes of SMTP Error 421 4.7.28
Now, the SMTP Error 421 4.7.28 is very common, but the things that lead to it vary. Sending patterns, authentication failure or reputation issues can cause this error, making your email to be treated by email service providers as suspicious.
1. Sending too many emails too quickly
Email providers monitor sending volume and frequency to detect potential spam. So if an IP suddenly starts sending a high number of emails in a short period, this raises a red flag. ESPs may temporarily block the sender to prevent spam attacks or phishing attempts. This is especially common for new domains, cold email campaigns, or recently activated mail servers.
2. High spam complaints & low engagement
If too many recipients mark your emails as spam, this tells ESPs that your messages are unwanted or unsolicited. Low open and click-through rates also give off signals that your emails lack engagement, making providers more likely to restrict your sending. Repeated poor engagement can quickly damage your sender reputation and lead to temporary blocks or even blacklisting.
🔖 Related Reading: Why are My Emails Going to Spam or Junk? [Solved]
3. Authentication failures (SPF, DKIM, DMARC issues)
Email authentication protocols—SPF, DKIM, and DMARC—help ESPs verify whether an email is coming from a trusted source. If these records are misconfigured, missing, or failing, your emails may be flagged as spoofed or fraudulent. Without proper authentication, major providers are more likely to block emails from your domain to protect their users.
4. Blacklisted or low-reputation IP Address
If your IP address is listed on a real-time blacklist (RBL), many ESPs may resort to automatic rejection of your emails. This often happens if your domain has a history of sending spam, high bounce rates, or poor email practices. Now, even if your own practices are clean, using a shared IP address can affect your reputation depending on what other users on the same network do.
5. Triggering spam filters
The ESPs inspect your email content, format, and structure to see if a message might be junk. Overpromotional words, too many images, too many links, or large attachments could trigger spam filters. Some words and phrases—such as “FREE!!! ” or “LIMITED TIME OFFER”—can raise the potential of emails being flagged, resulting in temporary blocks of the IP.
Step-by-step guide to fix SMTP 421 4.7.28
If your emails are being blocked due to SMTP Error 421 4.7.28, follow these steps to restore your sender reputation and regain inbox placement.
Step 1: Pause sending for a bit
Before attempting to fix the issue, stop all outgoing emails from the affected IP address or domain for at least 24 hours. Continuing to send emails while blocked can worsen your sender reputation and extend the restriction.
- Halt all email campaigns for at least a day to prevent further damage.
- Identify affected mailboxes and exclude them from sending temporarily.
- Monitor bounce logs to confirm when the issue started and which providers are rejecting your emails.
Step 2: Improve email authentication
Ensure these records are correctly configured to prove legitimacy to ESPs and prevent future rejections.
- Verify your SPF, DKIM, and DMARC records using tools like MXToolbox or Warmy.io’s free authentication checker.
- Ensure your SPF policy includes only authorized sending IPs to avoid failures.
- Set up BIMI (Brand Indicators for Message Identification) to improve sender trust.
- Enable TLS encryption to protect email transmissions and build trust with ESPs.
Step 3: Check & remove from blacklists
If your IP has been flagged, check if it appears on real-time blacklists (RBLs). Many ESPs rely on these lists to block potentially harmful senders. Delisting requests can speed up recovery and restore email delivery.
- Check your IP and domain reputation using Spamhaus, SURBL, and UCEPROTECT.
- Request removal from blacklists that offer delisting procedures.
- Review past sending practices to determine what triggered the listing and fix any issues before resuming sending.
🔖 Related Reading: How to check IP reputation? IP blacklists
Step 4: Implement warmup & gradual sending
Jumping back into full-scale sending too quickly can trigger another block. Instead, gradually increase email volume over days or weeks to rebuild trust with ESPs.
- Start with a small batch of emails (e.g., 50-100 per day) and increase gradually.
- Use Warmy.io’s AI-driven email warmup to automate the process and optimize inbox placement.
- Prioritize highly engaged recipients (those who regularly open and interact with your emails) to boost sender reputation.
Step 5: Monitor & adjust sending practices
Once sending resumes, it’s important to continuously monitor email performance and adjust based on feedback from ESPs. Implementing best practices prevents future blocks and ensures long-term deliverability.
- Segment your email list and remove inactive or disengaged users.
- Follow provider-specific sending limits, such as Gmail’s 500 emails/day for free accounts or 2,000/day for paid accounts.
- Monitor email performance metrics, including bounce rates, spam complaints, and inbox placement.
🔖 Related Reading: The Science and Process of Warming Up Newly Created Email Domains
Maximizing Warmy.io for preventing SMTP Error 421 4.7.28 and enhancing overall deliverability
Warmy.io is an AI-driven email deliverability platform designed to prevent, detect, and resolve SMTP errors like 421 4.7.28 before they impact your email campaigns.
By gradually warming up your email accounts, optimizing sending patterns, monitoring reputation, and automating deliverability checks, Warmy.io ensures that your emails land in the inbox rather than being blocked by ESPs. Its proactive approach helps businesses avoid rate limits, strengthen sender reputation, and maintain a high level of email authentication compliance—all essential for preventing temporary IP blocks and long-term blacklisting.
Domain health hub for monitoring & maintaining a strong sender reputation
Warmy.io’s Domain Health Hub is a centralized dashboard that helps you track and optimize your domain’s reputation in real time. Since SMTP Error 421 4.7.28 is often triggered by poor domain reputation, authentication failures, or spam complaints, this feature gives you full visibility into the factors that affect your deliverability—before they become a serious issue.
Here’s what the Domain Health Hub provides:
- Percentage of your emails going to the primary inbox, spam folder, or promotions tab.
- Spam rates, inbox placement, and deliverability trends on a weekly & monthly basis
- One dashboard to monitor deliverability at the domain level
- Comprehensive DNS status checks to easily validate SPF, DKIM, DMARC, rDNS, MX, and A records for stronger authentication & security
SPF, DKIM, and DMARC authentication checkers
Authentication failures are a major reason for SMTP 421 4.7.28 errors. Warmy.io automatically verifies your SPF, DKIM, and DMARC records, ensuring that your emails pass security checks and are not rejected.
- Free SPF Record Generator for generating SPF records to authenticate your email server
- Free DMARC Record Generator to help you create DMARC policies for protection of your domain
AI-powered email warmup and advanced seed lists to avoid sending rate triggers
Email providers flag sudden spikes in email volume as suspicious. Warmy.io gradually increases your sending activity to build trust with ESPs, preventing temporary IP blocks caused by sending too many emails too quickly. Here’s how Warmy does it:
- Warming up email accounts is done by mimicking natural email activity, including sending, replying, and marking emails as important.
- Capable of handling up to 5,000 emails per day, so it is definitely one of the most robust solutions on the market
- Supports custom templates, themes, and multiple languages to match user requirements.
- Tracks metrics like spam rates, open rates, and inbox placement.
Additionally, Warmy also offers advanced seed lists for greater deliverability success. By sending emails to a vetted list of seeds, senders can significantly improve sender reputation, reduce bounce rates, and even test different sending strategies to improve overall email engagement.
Compared to traditional email warmup, Warmy’s seed lists move beyond simulating engagement. It’s not just a simple list, it’s an opportunity to unlock the potential of email deliverability. Composed of genuine email addresses, Warmy’s advanced seed lists interact with emails and similar real recipient behavior—email opens, link clicking, scrolling, and even manually retrieving emails that land in spam and marking them as important.
Learn more about Warmy’s seed lists and how they fill in the gaps in traditional warmup.
Prevent SMTP errors & secure your email deliverability with Warmy.io
SMTP 421 4.7.28 errors can disrupt your email outreach, harm your sender reputation, and lead to long-term blacklisting if not managed properly. However, with the right preventative measures—including email warm-up, authentication monitoring, blacklist checks, and deliverability optimization—you can keep your emails landing in the inbox where they belong.
Warmy.io automates these critical processes, ensuring that you stay ahead of temporary IP blocks, spam filters, and reputation issues. Whether you’re recovering from an email block or looking to prevent future deliverability issues, Warmy provides real-time insights and AI-driven solutions to keep your email strategy running smoothly.
Don’t wait until your emails are blocked! Take control of your deliverability today with Warmy.io. Start your free trial today.
