{"id":4042,"date":"2025-03-25T12:32:00","date_gmt":"2025-03-25T12:32:00","guid":{"rendered":"https:\/\/www.warmy.io\/blog\/understanding-esp-blocks-causes-types-and-prevention\/"},"modified":"2026-04-01T14:31:14","modified_gmt":"2026-04-01T14:31:14","slug":"understanding-esp-blocks-causes-types-and-prevention","status":"publish","type":"post","link":"https:\/\/www.warmy.io\/blog\/understanding-esp-blocks-causes-types-and-prevention\/","title":{"rendered":"Understanding ESP Blocks: Causes, Types, and Prevention"},"content":{"rendered":"

Predictions show that by 2027, there will be <\/span>408.2 billion emails<\/span><\/a> 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.\u00a0<\/span><\/p>\n

That being said, email deliverability is the unsung hero behind every successful campaign.\u00a0<\/span><\/p>\n

However, amidst the hustle to craft the perfect email, most are blindsided by a common yet misunderstood hurdle\u2014<\/span>email blocks<\/i><\/b>.<\/span> These blocks can impede your efforts, sending your email to a <\/span>temporary deferral<\/span><\/a> status or a permanent spam folder placement\u2014ultimately damaging your sender reputation.<\/span><\/p>\n

Warmy\u2019s research team decided to take a deep dive into the inner workings of email blocks\u2014causes, types, and prevention strategies.<\/span><\/p>\n

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\u2019re 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\u2014in the inbox.<\/span><\/p>\n

What are ESP blocks?<\/h2>\n

Email Service Providers (ESPs)\u2002such as Gmail, Outlook, and G Suite (now Google Workspace) are at the core of controlling the emails that get to their users.<\/span><\/p>\n

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\u2002much more. So if an email looks suspicious or violates\u2002a provider\u2019s policies, it\u2019s blocked or sent to the spam folder.<\/span><\/p>\n

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\u2019s credibility even more with the ESP and they could find themselves facing even harsher blocks\u2002or a longer road to deliverability. Therein lies the importance of knowing why ESP blocks happen\u2014and\u2002how to avoid them\u2014for any business that depends on email marketing or outreach.<\/span><\/p>\n

Causes of email provider blocks<\/h2>\n

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\u2002blocked.<\/span><\/p>\n\n\n\n\n\n
\n

Cause<\/p>\n<\/th>\n

\n

Description<\/p>\n<\/th>\n<\/tr>\n<\/thead>\n

\n

Sender reputation (Domain\/IP) <\/p>\n<\/td>\n

\n

ESPs heavily favor senders with good reputations. Poor domain\/IP reputation leads to blocks or deferrals from providers <\/p>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

One of the questions many email senders have is:<\/span><\/p>\n

Q:<\/strong> What does sender reputation have to do with email deliverability?<\/span><\/i><\/p>\n

A:<\/strong> A good\u2002sender reputation is developed over time through your recipients\u2019 levels of engagement with your emails, your complaint rates, and your authentication practices, among other things.<\/span><\/i><\/p>\n\n\n\n\n\n
\n

Cause<\/p>\n<\/th>\n

\n

Description<\/p>\n<\/th>\n<\/tr>\n<\/thead>\n

\n

New domains or IPs <\/p>\n<\/td>\n

\n

Domains or IPs with no reputation are especially vulnerable and more prone to being blocked if they suddenly attempt high-volume sending. <\/p>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

What does this mean for new domains or IPs?<\/b> Warmy\u2019s 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: <\/span>The Science and Process of Warming Up Newly Created Email Domains<\/span><\/a>.<\/span><\/p>\n\n\n\n\n\n
\n

Cause<\/p>\n<\/th>\n

\n

Description<\/p>\n<\/th>\n<\/tr>\n<\/thead>\n

\n

Lack of or misconfigured authentication <\/p>\n<\/td>\n

\n

Failing SPF, DKIM, and DMARC authentication checks can cause ESPs to reject emails, thinking they are forged or suspicious. <\/p>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Sender Policy Framework (SPF), DomainKeys Identified Mail<\/span><\/p>\n

(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.\u00a0<\/span><\/p>\n

These protocols help prevent email spoofing and ensure that emails are sent from authorized sources\u2014all of which boosts trust with email providers and improves deliverability.\u00a0<\/span><\/p>\n

\ud83d\udd16 Related Reading:<\/b> SPF, DKIM, and DMARC: Boosting Email Security and Deliverability<\/span><\/a><\/p>\n\n\n\n\n\n
\n

Cause<\/p>\n<\/th>\n

\n

Description<\/p>\n<\/th>\n<\/tr>\n<\/thead>\n

\n

Spammy content and keywords <\/p>\n<\/td>\n

\n

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. <\/p>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

ESP filters are programmed to classify email spam depending on certain criteria. Though some businesses are legitimate, using wrong phrasings or \u201cspammy\u201d words can trigger these filters.<\/span><\/p>\n

\ud83d\udd16 Related Reading:<\/b> Why are My Emails Going to Spam or Junk? [Solved]<\/span><\/a><\/p>\n\n\n\n\n\n\n
\n

Cause<\/p>\n<\/th>\n

\n

Description<\/p>\n<\/th>\n<\/tr>\n<\/thead>\n

\n

Sudden volume spikes <\/p>\n<\/td>\n

\n

Sending too many emails too quickly can cause ESPs to treat sudden surges as potential spam outbreaks. <\/p>\n<\/td>\n<\/tr>\n

\n

Low engagement & spam complaints <\/p>\n<\/td>\n

\n

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. <\/p>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

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\u2019s important to know how to avoid spam complaints.<\/span><\/p>\n

\ud83d\udd16 Related Reading:<\/b> Spam Complaint Rate: What It Is and How to Reduce It<\/span><\/a><\/p>\n\n\n\n\n\n
\n

Cause<\/p>\n<\/th>\n

\n

Description<\/p>\n<\/th>\n<\/tr>\n<\/thead>\n

\n

Bad mailing lists (bounces & spam traps) <\/p>\n<\/td>\n

\n

High bounce rates or sending to stale, invalid addresses can get you flagged Spam traps\u2014inactive email addresses used to catch spammers\u2014can also cause severe deliverability issues. <\/p>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Types of blocks and how to identify them<\/h2>\n

Being aware of the different types of blocks email providers may implement can help you diagnose problems fast, and respond appropriately. Here\u2019s a rundown of the most common types of email blocks and tips on how to recognize them.<\/span><\/p>\n

\"A<\/p>\n

Reputation-based blocks<\/strong><\/h3>\n

Usually, reputation-based blocks are the result of a bad sender reputation,\u2002generally associated with the historical data of your domain or IP. If your\u2002emails always get marked as spam or you have low engagement, your messages may then be blocked or routed to spam folders.<\/span><\/p>\n

Volume-based blocks<\/strong><\/h3>\n

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.<\/span><\/p>\n

Many email senders or new businesses ask a critical question:<\/span><\/p>\n

Q: If we\u2019ll get blocked by ESPs for sending high email volumes, how do we maximize\u2002email marketing?<\/span><\/i><\/p>\n

A: The issue isn\u2019t necessarily sending high volumes of emails, but sending them without a proper warmup process. Building a positive sender reputation\u2002with 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\u2002warmup 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.<\/span><\/i><\/p>\n

Steps in identifying blocks<\/strong><\/h3>\n
    \n
  1. Check the SMTP code<\/b><\/a>. 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.<\/span><\/li>\n
  2. Read the bounce message<\/b>. The bounce-back message (Non-Delivery Report) often provides clues as to why the email was blocked. Look for keywords like \u201cdeferred,\u201d \u201crejected,\u201d \u201cblacklist,\u201d or \u201cblocked\u201d to help pinpoint the cause.<\/span><\/li>\n
  3. Check for blacklist listings<\/b>. If you think that your\u2002IP 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 <\/span>steps to get delisted<\/span><\/a>.<\/span><\/li>\n
  4. Analyze your sending behavior<\/b>. 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.<\/span><\/li>\n
  5. Use monitoring tools<\/b>. Services such as Gmail Postmaster and Microsoft\u2019s SNDS offer useful insights into your email reputation, so you can monitor any issues before they become ESP blocks.<\/span><\/li>\n<\/ol>\n

    How to alleviate and prevent blocks<\/h2>\n

    Below are key strategies to alleviate and prevent blocks:<\/span><\/p>\n

    Warm up your domain and IP<\/strong><\/h3>\n