What should I do when my IP gets blocklisted during IP warmup?

Key findings

• Data quality is key: Even with purchased lists or existing customers, issues like typos, made-up addresses, or subtle spam traps can exist and lead to blocklists. Recycling lists, even of active customers, can increase the risk if engagement is not consistently high.
• Engagement matters: ISPs (Internet Service Providers) like Microsoft monitor recipient interactions. If emails are consistently deleted without reading, marked as spam, or ignored, it negatively impacts sender reputation, potentially leading to bounces or blocklisting.
• Direct blocklisting: While an IP might be blocklisted by one provider (e.g., Comcast), this information doesn't typically propagate directly to others like Microsoft. Instead, a separate trigger (e.g., spam complaints, low engagement, or hitting internal spam filters) at Microsoft's end likely caused their block.
• ISP-specific remediation: Each ISP and blocklist operator has its own process for delisting. It's often necessary to contact them directly, address the underlying issues, and request removal. Automated delisting periods can vary.
• Warmup strategy: IP warmup is about gradually building trust. Any deviation from the plan, or a sudden change in sending behavior or list quality, can derail the process and lead to blocklisting. Consistent, engaged sends are paramount for a healthy IP warmup.

Key considerations

• Immediate action: Temporarily halt sending to affected domains (like Microsoft in this case) from the blocklisted IP. This prevents further damage while you investigate and remediate.
• Root cause analysis: Identify what specifically triggered the blocklist. Was it a sudden spike in complaints, a content issue, or hitting a specific spam trap? Checking why your IP was flagged can provide clues.
• Engagement strategy: Re-evaluate your segmentation strategy, especially if recycling lists. Prioritize sending to your most engaged subscribers to build positive reputation before expanding to less active ones. Consider implementing double opt-in.
• ISP communication: Contact Microsoft support or their postmaster team directly to understand the blocking reason and initiate the delisting process. Be prepared to provide details about your sending practices and remediation efforts.
• Monitor and learn: Continuously monitor your IP and domain reputation. Use feedback loops and bounce codes to identify issues early. This can help prevent the second IP from facing similar issues, especially since they share the same domain and network. For more information on dealing with other blacklists, consider exploring specific guides.

Key opinions

• List quality misconception: Many marketers believe their lists are clean because they consist of purchasers or active clients, but issues like typos, abandoned addresses becoming spam traps, or general disengagement can still lead to problems during warmup.
• Engagement-focused warmup: The consensus is that IP warmup should primarily focus on sending to the most engaged segments of your audience first, gradually expanding to less active users. This helps build a positive sending history with ISPs.
• ISP-specific blocklisting: Marketers frequently note that a blocklist by one ISP (e.g., Comcast) does not necessarily imply direct propagation to another (e.g., Microsoft). Each ISP makes its own decisions based on its internal spam filters and received signals.
• Recycling lists risks: Repeatedly sending to the same list segments, even if they were responsive initially, can lead to recipient fatigue and increased complaints, especially if the content isn't consistently highly engaging.
• ESP role: While ESPs (Email Service Providers) like Salesforce Marketing Cloud should ideally manage IP health, marketers often find themselves needing to take proactive steps and directly engage with ISPs when issues arise.

Key considerations

• Analyze subscriber behavior: Beyond list source, it's crucial to analyze how subscribers are interacting with your emails. Low opens, high deletes-without-reading, or direct spam complaints (even if small in number) signal issues to ISPs.
• Content relevance: Ensure your email content remains highly relevant and desired by the audience you're sending to. Generic content or a sudden shift in message can lead to negative engagement signals.
• Proactive mitigation: If bounces or blocklists occur, immediately pause sending to the affected domains or ISPs from the problematic IP. This minimizes further reputational damage. Remember that understanding why your IP is blacklisted is the first step to fixing it.
• Direct communication for delisting: Utilize postmaster tools and direct contact forms provided by ISPs (like Microsoft) to request delisting. Be transparent about your efforts to resolve the issue. Inboxy provides a guide to removing your IP from blacklists which can be helpful.
• Double opt-in consideration: While not always feasible for existing customer lists, implementing double opt-in for new subscribers can significantly reduce spam trap hits and improve overall list quality, preventing future blocklists.

Key opinions

• Independent ISP policies: Experts confirm that major ISPs like Comcast and Microsoft generally do not share delivery or blocklisting information directly with each other. A block by one (e.g., Comcast) does not cause a block by another (e.g., Microsoft).
• Internal spam filters: Microsoft's blocklisting is likely due to their internal spam filters being triggered by the specific email volume or content, or by negative engagement signals received from their users. High rate limiting (4xx errors) during warmup can also be a precursor to a block.
• Data hygiene is paramount: Even for lists of active purchasers, there's a non-zero risk of encountering spam traps or invalid addresses. Relying solely on purchase history without continuous validation can be problematic, especially when scaling volume during warmup. Understanding how spam traps work is critical.
• Engagement dictates reputation: Sender reputation is largely built on how recipients interact with emails. High complaint rates, low opens, or deletions without opening can quickly erode trust, leading to blocklists, even for legitimate senders. Learn how to boost email deliverability rates.
• Direct remediation is often necessary: When a major ISP blocks an IP, the most effective course of action is to directly follow their bounce message URLs, access their postmaster tools, or contact their support for delisting. Waiting for automatic removal is not a reliable strategy.

Key considerations

• Isolate the problematic IP: Temporarily stop sending from the blocklisted IP to the affected ISP (e.g., Microsoft). Focus on the healthy IP to continue building its reputation while the other is being remediated.
• Address underlying causes: Beyond delisting, investigate what triggered the block. This might involve re-evaluating content, reviewing engagement metrics for the specific segments sent to, and enhancing list validation processes to prevent future occurrences.
• Leverage postmaster tools: Sign up for and actively monitor postmaster tools provided by major ISPs (e.g., Google Postmaster Tools, Outlook.com Postmaster). These tools offer invaluable data on your sender reputation, spam rates, and specific blocking reasons.
• Understand rate limiting: If you observe 4xx errors (temporary failures) during warmup, it's a sign that you might be sending too fast or have content issues. Address these promptly to avoid escalating to hard blocks or blocklisting. See factors that influence IP reputation.
• Don't assume propagation: Just because one IP is blocked, don't automatically assume the adjacent IP will follow. While they share characteristics, each IP's reputation is built individually. Continue monitoring the healthy IP vigilantly and ensure its sending practices remain optimal.

Key findings

• Gradual reputation building: IP warmup is designed to slowly build trust with ISPs by demonstrating consistent, positive sending behavior. Sending too much too soon, or to unengaged audiences, can quickly backfire. See IP warming tips.
• Engagement is critical: ISPs monitor how recipients interact with emails (opens, clicks, deletes, spam complaints). Low engagement or high negative feedback (even during warmup) will harm your IP's reputation.
• List hygiene: Even opt-in lists can contain invalid addresses or spam traps over time. Regular list cleaning and validation are essential to prevent hitting blocklists. This includes preventing different types of spam traps from causing issues.
• Blocklist diversity: There are hundreds of blacklists, both public and private. Being listed on one does not automatically mean you're on all of them, nor does it guarantee direct propagation between major ISPs. An in-depth guide to email blocklists can provide more context.
• Remediation processes: Most blocklists provide a delisting process, often requiring the sender to identify and fix the cause of the listing. This often means engaging with ISP postmaster sites or specific blocklist removal pages.

Key considerations

• Pause and investigate: Upon detecting a blocklist, immediately pause sending to the affected recipients from that IP. Analyze recent sending logs for bounce codes and error messages, which often provide clues to the cause (e.g., spam complaints, unknown users).
• Identify specific blocklists: Use online blocklist checkers to identify which specific blacklists have listed your IP. This helps prioritize remediation efforts and understand the scope of the problem. What happens when your IP gets blocklisted can vary significantly by blocklist.
• Remediate the root cause: Before requesting delisting, ensure the underlying issue (e.g., poor list quality, high complaint rates, spammy content) has been addressed. Without this, you risk being re-listed quickly.
• Consult ISP postmaster pages: Major ISPs (like Microsoft) have dedicated postmaster sites with guidelines for senders, troubleshooting tools, and specific forms for delisting requests. Following their documented procedures is the most effective path to resolution.
• Adjust warmup strategy: If a blocklist occurs during warmup, it's a strong signal to re-evaluate and potentially adjust your warmup schedule, focusing more on quality and engagement over rapid volume increases.