What should I do when my IP gets blocklisted during IP warmup?
Michael Ko
Co-founder & CEO, Suped
Published 3 Jun 2025
Updated 16 Aug 2025
8 min read
Discovering that your IP has been blocklisted during the critical IP warmup phase can be incredibly frustrating. This period is dedicated to building a positive sender reputation with internet service providers (ISPs), and a sudden blocklist (or blacklist) entry feels like a major setback. It means your carefully planned sending schedule is disrupted, and your emails might not reach their intended recipients.
The goal of IP warmup is to gradually increase your sending volume, allowing ISPs to observe your sending patterns and gauge subscriber engagement. This slow and steady approach helps prove you are a legitimate sender, not a spambot. When an IP gets blocklisted, it often signals an underlying issue that needs immediate attention, otherwise your email deliverability will suffer.
I'll guide you through the necessary steps to diagnose the problem, remove your IP from the blocklist, and adjust your warmup strategy to prevent future occurrences. Remember, a blocklist is a sign that an ISP or a blocklist provider has detected suspicious activity, and a swift, informed response is crucial.
When your IP address is blocklisted (or blacklisted) during warmup, your first priority is to halt all sending from that IP immediately. Continuing to send will only worsen the situation and dig you deeper into a reputational hole. Next, you need to identify exactly where your IP is listed and understand the specific bounce messages you're receiving.
Check bounce messages: Analyze the full bounce error codes. They often provide clues, such as mentioning a specific blocklist (e.g., 550 5.7.1 Service unavailable) or pointing to a postmaster page for more information. For instance, Microsoft error codes can indicate a block. If you are experiencing issues specifically with Microsoft, consult our article on how to handle Microsoft blocks.
Use a blocklist checker: Input your IP address into a reputable blocklist checker. This will show you which specific DNS-based Blackhole Lists (DNSBLs) have listed your IP. Common ones include Spamhaus, SpamCop, and Proofpoint. For an in-depth understanding, see our guide on how email blacklists actually work.
Knowing which blocklist (or blacklist) you're on is crucial because each one has its own delisting process. Some are automated and temporary, while others require manual requests and evidence of corrective action. Understanding the type of blocklist will help you determine the best path forward.
Addressing the root cause of the blocklist
A blocklist entry, especially during warmup, usually indicates an issue with your sending practices or list quality. Even if you believe your list is clean, subtle problems can arise. Here are the common culprits:
Spam traps: Even customer lists can contain spam traps. These are email addresses designed to catch spammers. Hitting one during warmup (when your reputation is still fragile) can quickly lead to a blocklisting. People mistype email addresses or abandon old ones that later become traps. For further details, consider reading our content on different types of spam traps.
High complaints/low engagement: If recipients mark your emails as spam, delete them without reading, or show little engagement, ISPs will view your mail negatively. During warmup, it's vital to send highly engaging content to your most active subscribers first. This is a common reason for Gmail spam issues.
Sudden volume spikes: While a warmup plan involves increasing volume, too sharp an increase, especially to a less engaged segment, can trigger spam filters and lead to a blocklist or (blacklist) entry. It's why dedicated IP warm-up best practices emphasize gradual escalation.
Symptoms to look for
You might see increased bounce rates, particularly from specific ISPs like Comcast.com or Microsoft.com domains. Delivery delays or outright rejections are also common. Pay close attention to any error messages that accompany these delivery failures, as they are often very specific about the reason for the block. A block on one ISP, like Comcast, does not necessarily propagate directly to others, such as Microsoft. ISPs often use their own internal blocklists or external real-time blocklists (RBLs) that they subscribe to. So, a block with one ISP might just be a symptom of a broader reputation issue that other ISPs are also detecting independently.
Delisting and communication
Once you've identified the blocklist(s) and paused sending, your next step is to initiate the delisting process. This often involves direct communication with the ISP or blocklist operator, coupled with demonstrating corrective action.
Contact the blocklist operator: Visit the website of the blocklist (or blacklist) provider, such as Spamhaus or UCEPROTECT, and use their lookup tool to confirm your IP's listing. Most will have a form or instructions for requesting delisting. Be prepared to explain your situation, your measures to prevent recurrence, and your IP warm-up strategy. This is a specific step for Spamhaus blocklist removals.
Engage your email service provider (ESP): If you're using a dedicated IP through an ESP (like Salesforce Marketing Cloud), they should assist with delisting. They often have established relationships with ISPs and blocklist operators, which can expedite the process. However, be aware that some ESPs might be slow to react, so proactive monitoring on your part is important.
Implement a cool-down period: It's often beneficial to implement a "cool-down" period for the blocklisted IP, especially for the affected domains (e.g., Comcast or Microsoft). This allows the IP's reputation to stabilize and demonstrates to ISPs that you've addressed the issue. This period can vary, but generally, 24-72 hours or even longer is advisable depending on the severity of the block and the blocklist involved.
Delisting process comparison
Automated blocklists: Some blocklists (e.g., SORBS, UCEPROTECT) automatically delist IPs after a certain period (e.g., 24-72 hours) if no further spam activity is detected. Your primary action here is to fix the underlying issue to prevent re-listing.
Manual review blocklists
Reputation-based lists: Others, particularly ISP internal blocklists (like Microsoft.com or Gmail.com), require a manual request. You'll need to fill out a delisting form and provide details about your sending practices, list acquisition methods, and how you plan to improve your sending hygiene. Proof of a clean list and good engagement metrics will be vital here, especially when recovering from Gmail blocks.
Prevention and ongoing strategy
A blocklist during warmup highlights the need for careful ongoing management of your email program. Even after delisting, you must adjust your IP warm-up strategy to avoid recurrence and build a robust sender reputation.
Segment by engagement: During IP warmup, prioritize sending to your most engaged subscribers first. These are the ones who consistently open, click, and interact with your emails. This sends positive signals to ISPs. Gradually introduce less engaged segments as your IP reputation grows. This is especially important when you warm up new IP addresses.
Implement double opt-in: While you mentioned having a clean list of existing customers, implementing double opt-in for new subscribers significantly reduces the risk of spam traps and unengaged users, enhancing your email deliverability. This practice ensures that every subscriber explicitly confirms their desire to receive your emails, which is a strong positive signal to ISPs. Learn more about email deliverability issues.
Regularly monitor your IP and domain reputation. Services that provide blocklist monitoring can give you real-time alerts if your IP appears on a blacklist. This proactive approach allows you to react quickly before a minor issue escalates into a widespread block.
Views from the trenches
Warming up a new IP or recovering from a blocklist (or blacklist) often involves subtle factors that aren't immediately obvious. Here are some insights from experts and marketers in the field.
Best practices
Always segment your list by engagement level, starting your warmup with the most active subscribers.
Use double opt-in for all new subscribers to ensure high-quality, engaged contacts.
Monitor your IP and domain reputation daily, including checking bounce codes and blocklist status.
Tailor your content to be highly engaging during warmup, focusing on valuable information rather than aggressive sales.
Common pitfalls
Assuming purchased lists are free of spam traps, even if they're from existing customers.
Increasing sending volume too quickly or to unengaged segments during the later stages of warmup.
Ignoring subtle warning signs like increased rate limiting before a full block occurs.
Believing that one ISP's blocklist automatically propagates to all others.
Expert tips
Engage your ESP's support team immediately when a blocklist occurs, as they often have direct channels to ISPs for delisting.
Be patient, as delisting and reputation recovery can take time, sometimes weeks depending on the severity.
If possible, analyze engagement metrics (opens, clicks, complaints) for the specific sending period leading to the blocklist.
Consider reducing volume or pausing sends to the affected ISP if a block persists, then slowly reintroduce after delisting.
Expert view
Expert from Email Geeks says that a blocklist during warmup is most likely due to a data quality issue, suggesting to investigate the subscriber list used in the last email group.
2022-09-14 - Email Geeks
Expert view
Expert from Email Geeks says that IP warmup is about introducing yourself as a valid sender to the ISP, and you should target active users for this process.
2022-09-14 - Email Geeks
Moving forward and building reputation
Dealing with an IP blocklist during email warmup is a challenging but recoverable situation. The key is a systematic approach: immediately pausing sends, thoroughly diagnosing the cause, engaging with blocklist operators and your ESP, and critically, adjusting your sending practices to prevent future issues.
Remember that IP warmup is an ongoing process of building trust. A block (or blacklist) is a signal, not a permanent condemnation. By taking decisive action and refining your approach to list segmentation and content, you can restore your IP's reputation and ensure your emails consistently reach the inbox.
For more information on why your emails might be going to spam, check out our comprehensive guide: Why Your Emails Fail.
Spamhaus
0Spam
Cisco
NoSolicitado
URIBL
abuse.ro
ALPHANET
Anonmails
Ascams
BLOCKEDSERVERS
Calivent Networks
EFnet
JustSpam
Kempt.net
NordSpam
RV-SOFT Technology
Scientific Spam
Spamikaze
SpamRATS
SPFBL
Suomispam
System 5 Hosting
Team Cymru
Validity
www.blocklist.de Fail2Ban-Reporting Service
ZapBL
2stepback.dk
Fayntic Services
ORB UK
technoirc.org
TechTheft
