Predictions show that by 2027, there will be 408.2 billion emails sent per day. What does this mean? It shows how big a role email has in everyday life. For businesses and individuals depending on email as primary means of communication, the ability to land in the inbox is everything.
That being said, email deliverability is the unsung hero behind every successful campaign.
However, amidst the hustle to craft the perfect email, most are blindsided by a common yet misunderstood hurdle—email blocks. These blocks can impede your efforts, sending your email to a temporary deferral status or a permanent spam folder placement—ultimately damaging your sender reputation.
Warmy’s research team decided to take a deep dive into the inner workings of email blocks—causes, types, and prevention strategies.
But this report is more than just an overview. It is carefully tailored to support Warmy users in maximizing their email deliverability. So whether you’re new to the concept or have experienced the frustration of blocked emails, this report will empower you with the knowledge to keep your outreach smooth, effective, and, most importantly—in the inbox.
What are ESP blocks?
Email Service Providers (ESPs) such as Gmail, Outlook, and G Suite (now Google Workspace) are at the core of controlling the emails that get to their users.
They rely on sophisticated algorithms and filters to decide which emails get delivered to the inbox and which are marked as spam or, even worse, blocked altogether. This decision is based on sender reputation, email content, engagement rates, and much more. So if an email looks suspicious or violates a provider’s policies, it’s blocked or sent to the spam folder.
This can be a problem for businesses. If your emails are blocked, your attempts to reach out to prospects, customers, or partners are impacted. Even worse, if these blocks occur repeatedly, it damages the sender’s credibility even more with the ESP and they could find themselves facing even harsher blocks or a longer road to deliverability. Therein lies the importance of knowing why ESP blocks happen—and how to avoid them—for any business that depends on email marketing or outreach.
Causes of email provider blocks
Here are some of the reasons why email providers block you. All of these factors are critical to impact whether your emails end up in the inbox or get blocked.
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Cause
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Description
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Sender reputation (Domain/IP)
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ESPs heavily favor senders with good reputations. Poor domain/IP reputation leads to blocks or deferrals from providers
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One of the questions many email senders have is:
Q: What does sender reputation have to do with email deliverability?
A: A good sender reputation is developed over time through your recipients’ levels of engagement with your emails, your complaint rates, and your authentication practices, among other things.
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Cause
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Description
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New domains or IPs
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Domains or IPs with no reputation are especially vulnerable and more prone to being blocked if they suddenly attempt high-volume sending.
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What does this mean for new domains or IPs? Warmy’s research team conducted a study to determine how different warm-up strategies affect the deliverability of emails from newly created domains. You can check the whole report here: The Science and Process of Warming Up Newly Created Email Domains.
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Cause
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Description
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Lack of or misconfigured authentication
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Failing SPF, DKIM, and DMARC authentication checks can cause ESPs to reject emails, thinking they are forged or suspicious.
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Sender Policy Framework (SPF), DomainKeys Identified Mail
(DKIM), and Domain-based Message Authentication, Reporting, and Conformance (DMARC) are email authentication protocols that work together to verify the legitimacy of an email sender.
These protocols help prevent email spoofing and ensure that emails are sent from authorized sources—all of which boosts trust with email providers and improves deliverability.
🔖 Related Reading: SPF, DKIM, and DMARC: Boosting Email Security and Deliverability
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Cause
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Description
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Spammy content and keywords
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Certain content and trigger keywords, or overuse of all-caps, can trigger spam filters. Modern filters now look at the overall context, including sender reputation and user engagement.
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ESP filters are programmed to classify email spam depending on certain criteria. Though some businesses are legitimate, using wrong phrasings or “spammy” words can trigger these filters.
🔖 Related Reading: Why are My Emails Going to Spam or Junk? [Solved]
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Cause
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Description
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Sudden volume spikes
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Sending too many emails too quickly can cause ESPs to treat sudden surges as potential spam outbreaks.
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Low engagement & spam complaints
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Engagement metrics like opens, replies, and clicks significantly impact deliverability. High spam complaints or low engagement can cause your emails to be flagged or diverted to spam folders.
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ESPs put a premium on what recipients think. So if you get spam complaints regularly, ESPs will label you as such. Aside from getting the technical details correct, it’s important to know how to avoid spam complaints.
🔖 Related Reading: Spam Complaint Rate: What It Is and How to Reduce It
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Cause
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Description
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Bad mailing lists (bounces & spam traps)
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High bounce rates or sending to stale, invalid addresses can get you flagged Spam traps—inactive email addresses used to catch spammers—can also cause severe deliverability issues.
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Types of blocks and how to identify them
Being aware of the different types of blocks email providers may implement can help you diagnose problems fast, and respond appropriately. Here’s a rundown of the most common types of email blocks and tips on how to recognize them.
Reputation-based blocks
Usually, reputation-based blocks are the result of a bad sender reputation, generally associated with the historical data of your domain or IP. If your emails always get marked as spam or you have low engagement, your messages may then be blocked or routed to spam folders.
Volume-based blocks
Volume-based blocks happen when you send too many emails in a short period. If you suddenly increase your email volume without a warmup process, providers may flag your account and temporarily block emails.
Many email senders or new businesses ask a critical question:
Q: If we’ll get blocked by ESPs for sending high email volumes, how do we maximize email marketing?
A: The issue isn’t necessarily sending high volumes of emails, but sending them without a proper warmup process. Building a positive sender reputation with email providers and gradually increasing your sending volume is a must to make the most out of your email marketing. New domains and IPs need time to build a reputation, and this warmup process helps your domain/IP get established rather than be blocked as an ESP. Consistent, strategic volume growth combined with proper authentication and engagement will allow you to scale your email marketing effectively while maintaining deliverability.
Steps in identifying blocks
- Check the SMTP code. Look at the bounce-back error code to determine whether the block is temporary (4XX) or permanent (5XX). A temporary block means you should pause and try again later, while a permanent block requires addressing the root cause.
- Read the bounce message. The bounce-back message (Non-Delivery Report) often provides clues as to why the email was blocked. Look for keywords like “deferred,” “rejected,” “blacklist,” or “blocked” to help pinpoint the cause.
- Check for blacklist listings. If you think that your IP or domain is being blocked due to its reputation, you can check popular blacklists such as Spamhaus or Barracuda. You can use tools to check whether your domain/IP is listed. You can then follow these steps to get delisted.
- Analyze your sending behavior. Review your email volume, frequency, content, and engagement rates. See if there are patterns of low engagement or sudden spikes in volume that may have triggered the block.
- Use monitoring tools. Services such as Gmail Postmaster and Microsoft’s SNDS offer useful insights into your email reputation, so you can monitor any issues before they become ESP blocks.
How to alleviate and prevent blocks
Below are key strategies to alleviate and prevent blocks:
Warm up your domain and IP
- Start by sending a small number of emails, and gradually increase the volume over time.
- Use Warmy.io for automated warmup. Warmy ensures your domain gains a good reputation without risking blocks due to sudden high volume.
Follow proper email authentication practices (SPF, DKIM, DMARC)
- Configure your SPF record to specify which mail servers are authorized to send emails on behalf of your domain. Here’s a helpful resource on The Definitive Guide to SPF in Email to get you started.
- Implement DKIM to add a cryptographic signature to your emails, which email providers can verify.
- Configure DMARC to define how ESPs should handle emails that fail SPF or DKIM checks and provide you with reports on email authentication results.
Optimize email content
- Stay away from common spam triggers, such as “free money,” too much punctuation (e.g., “!!! ”) or even misleading subject lines that can get your email tagged as spam.
- Always write emails in a professional tone, and personalize the content to make it more relevant to the recipient.
- Avoid using too many links or attachments. If you need to include links, ensure they are to reputable domains. Large attachments, especially file types like .exe or .zip, should be avoided in initial outreach emails.
- Always include an “unsubscribe” option, especially for bulk emails. This is legally mandated in the CAN-SPAM and GDPR.
- Keep HTML simple or use plain text. Plain text emails have fewer deliverability issues. We suggest using more text than images. There is actually an interesting finding about this in of our latest research reports: Inbox vs. Spam: How DNS Configuration & Email Templates Affect Your Email Success
Follow sending best practices
- Maintain a steady send volume and avoid sudden spikes in email volume. Don’t go from 50 in one day to suddenly 5,000 the next.
- Deliver valuable content to recipients who are likely to engage with your emails. High engagement rates (opens, clicks, and replies) signal to ESPs that your emails are wanted and relevant.
- Clean your mailing lists regularly by removing invalid email addresses and unsubscribe those who are no longer interested. Sending to bad or unengaged contacts can result in high bounce rates and spam complaints, which can harm your reputation.
- Respect ESP-specific limits. Different email providers have different sending limits, especially for new accounts. Be mindful of these limits and avoid exceeding them.
How to respond to a block
Now, what if your emails are already blocked? Well, ignoring the block or continuing to send emails without making changes can result in permanent damage to your sender reputation.
Here’s a step-by-step guide on how to respond to an email block:
- Stop and analyze. Identify the root cause of the block by reviewing your email authentication, sending volume, content, and list. Address the underlying issue to avoid recurring blocks.
- Wait if temporary. For temporary blocks (4XX errors), pause sending for 24-72 hours. Use this time to implement improvements (e.g., reducing send rates or authenticating your domain). Resuming without changes may lead to permanent blocks.
- Reduce volume and frequency. When resuming, start with a smaller email volume and gradually increase it.
- Change up your content. If a specific email triggered the block, avoid re-sending the same content. Modify the subject line, content, and links to prevent spam filters from flagging your messages again.
- Check and fix authentication. Ensure that your SPF, DKIM, and DMARC settings are correctly configured.
- Investigate blacklists. If your email is listed on blacklists (e.g., Spamhaus or Barracuda), request removal after resolving the cause of the block. Being delisted can take a few hours to days but is essential for recovery.
- Use provider postmaster support. Reach out to the ESP’s support (e.g., Gmail, Microsoft, Yahoo) for guidance on resolving the block. Many providers offer delisting portals or support forms.
- Rotate to a new domain/IP as a last resort. If the block is permanent and your domain reputation is severely damaged, you may need to switch to a new domain or IP. This should be a last resort and only after addressing the issues that led to the block.
Comparison of email providers’ filtering (Gmail vs. Outlook/Office365 vs. G Suite)
This comparison table provides a side-by-side look at how these three popular email services filter and block emails, with specific guidelines for bulk senders, throttling limits, and recovery processes.
Some major comparison points include:
- Gmail might silently categorize emails as spam if they are borderline “spammy” while Outlook might actually resort to bouncing them
- Gmail issues take longer to fix because it’s all about strategizing, but Outlook gives concrete instructions and steps
- Both Gmail and Outlook treat spam complaints as critical
- Outlook is very sensitive to sender reputation and list quality
- Both Outlook and Gmail now strongly enforce having proper DKIM/SPF
In a recent report by Warmy’s research team, we observed provider-specific deliverability trends,which impact inbox placement. We go deeper into this topic and you can access the report here: Inbox vs. Spam: How DNS Configuration & Email Templates Affect Your Email Success.
Warmy’s role in helping senders avoid and prevent ESP blocks
Warmy.io plays a pivotal role in helping businesses avoid and prevent email blocks, ensuring that email campaigns reach the inbox consistently. Here are some of Warmy’s key features that help prevent blocks:
Automated warmup process done right
Warmy gradually warms up new email domains and IP addresses, building a positive sender reputation over time. This process involves sending increasing volumes of emails to real inboxes, simulating authentic engagement, and establishing credibility with email providers. This helps avoid sudden volume spikes, a common trigger for email blocks.
Additional warmup preferences to maximize inbox placement
Warmup Preferences is a new feature from Warmy designed to help senders customize and fully control the warmup process from both sender and user levels.
In a nutshell, senders will be able to customize the warmup’s distribution across different providers. They can also choose if they want to use B2B or B2C customers for engagement patterns to tailor the behavior and insights to their business type. All of these settings can be changed right within the Warmy system for hassle-free customer experience.
Email deliverability tests and regular domain health monitoring
Aside from including checks for email authentication settings, Warmy’s free deliverability test provides inbox placement information on where your emails land. The test also helps identify any appearances on blacklists so senders can act right away.
Meanwhile, Warmy’s Domain Health Hub helps track and monitor domain reputation from one dashboard:
- Domain health score based on factors like authentication, blacklist status, and inbox placement.
- Spam rate trends and overall deliverability performance (weekly and monthly)
- DNS checks to validate SPF, DKIM, DMARC records for extra security
- Multi-domain monitoring for convenient tracking of all domains
- Reports on performance and other health metrics
Advanced seed lists
Warmy.io’s seed list feature can help businesses test and optimize email campaigns before sending bulk email campaigns to their actual recipients. This ensures high deliverability and low bounce rates.
Plus, while traditional warming methods lack engagement tracking, Warmy’s seed list uses real email addresses that actively interact with your emails. Specifically, this means:
- Your email is not just marked as read but actually opened and scrolled through
- If your email has any links, our system will click them naturally to improve sender trust.
- If an email finds its way in spam, it is manually removed and marked as important to let ISPs know your emails are legitimate.
- It can also be integrated into any email client to improve your email performance testing. Warmy will supply genuine email addresses from Google, Outlook, and Yahoo that will act as seeds for testing your email deliverability.
🔖 Related Reading: Email Warmup Alternatives: When Traditional Warmup Isn’t Enough
Credible email authentication checks
Warmy’s free deliverability test includes checks for email authentication settings such as SPF, DKIM, and DMARC, ensuring that your emails are properly authenticated and more likely to reach the inbox.
Additionally, Warmy has a free SPF Record Generator and free DMARC Record Generator to assist senders.
Spam template checker to help optimize your content
Warmy helps ensure that your emails bypass spam filters by optimizing email content. The free Template Checker can analyze your email content and warn you on any possibilities if your email will be tagged as spam. You can then adjust your content until it is fully optimized for sending.
Don’t let ESP blocks stop you from reaching your target customers
Ready to take your email deliverability to the next level? Book a demo with Warmy today and discover how we can help you optimize your sender reputation, avoid blocks, and ensure your emails reach the inbox.
Our team will walk you through how Warmy’s AI-powered features can transform your email campaigns, improve engagement, and boost your overall deliverability. Don’t let email restrictions hold you back—let Warmy help you maximize your email marketing success!
