Why is my spam complaint rate so high during email warm-up?
Michael Ko
Co-founder & CEO, Suped
Published 11 Jun 2025
Updated 17 Aug 2025
7 min read
Starting a new sending domain or IP address involves a crucial process known as email warm-up. This gradual increase in sending volume helps mailbox providers like Gmail and Outlook to recognize your sending patterns as legitimate, preventing your emails from being flagged as spam. It is a necessary step to build sender reputation and ensure high deliverability rates.
However, even with the best intentions and a carefully planned warm-up strategy, it is not uncommon to see a sudden spike in your spam complaint rate. This can be alarming, especially when you are trying to establish a positive sending history. The goal is always to keep your spam complaint rate as low as possible, ideally below 0.1%.
A high spam complaint rate during warm-up can derail your efforts, leading to blocklistings and long-term deliverability issues. Understanding why these spikes occur and how to address them is crucial for maintaining a healthy sender reputation and ensuring your messages reach the inbox.
Understanding the numbers
A spam complaint rate indicates the percentage of recipients who mark your email as spam relative to the number of emails delivered. Mailbox providers, such as Google, calculate this rate based on spam complaints from active users for inboxed emails. This specific definition is critical, especially during the warm-up phase.
During warm-up, your sending volume is intentionally low. If you send a very small number of emails and even one or two recipients mark your message as spam, the complaint rate can appear disproportionately high. For example, if you send 100 emails and only 10 actually land in the inbox for active users, a single complaint would result in a 10% spam complaint rate (1 complaint / 10 inboxed emails = 10%). If even fewer emails are inboxed, the percentage can soar dramatically, sometimes even exceeding 100% in reporting tools like Google Postmaster Tools, if the reported complaints exceed the number of emails that truly inbox that day.
It is important to remember that these spikes, particularly in the very early stages of warming, might be more about the mathematical calculation on a tiny volume than a widespread rejection of your emails. However, consistent high rates over several days will still lead to serious deliverability issues and potentially put your domain on a blacklist (or blocklist).
Why complaints spike during warm-up
Several factors can contribute to a high spam complaint rate during email warm-up, even when you are following best practices. These issues are often exacerbated by the sensitive nature of the warm-up period, where mailbox providers are closely scrutinizing your sending patterns to determine your sender reputation.
One of the primary culprits is list quality. If you are warming up an IP or domain using an unverified or outdated email list, you risk hitting spam traps or sending to disengaged users. Spam traps are email addresses designed to catch spammers, and hitting them immediately damages your sender reputation. Similarly, sending to recipients who did not explicitly opt-in or have not engaged with your emails in a long time can lead to a significant number of spam complaints. This is why sending to unengaged users during email warm-up is a major pitfall.
Another factor is the content of your emails. During warm-up, it is best to send highly engaging and expected content. If your emails are perceived as irrelevant, misleading, or promotional by recipients who were expecting something else, they are more likely to mark them as spam. Even subtle cues, like spammy subject lines or too many images, can trigger filters and recipient complaints, which can be seen in tools to reduce a high spam complaint rate. This is why it is often recommended to start warm-up with transactional or highly anticipated messages.
Finally, deviations from your warm-up schedule can cause issues. Sudden spikes in sending volume or inconsistent sending patterns can alert spam filters. Warm-up requires a consistent, gradual approach to build trust with mailbox providers. Any deviation might lead to a significant increase in spam complaints, pushing your emails to the junk folder during Microsoft IP warm-up, even with seemingly good metrics.
Common warm-up mistakes
Using old or unverified lists: Sending to disengaged recipients or spam traps that can immediately flag your domain or IP as suspicious.
Sending irrelevant content: Content that does not meet recipient expectations, leading to higher complaint rates.
Aggressive scaling: Increasing volume too quickly, which triggers spam filters because it mimics spammer behavior.
Ignoring early warning signs: Failing to monitor metrics closely, allowing issues to escalate before detection.
Best practices for warm-up
Clean your lists: Only send to highly engaged, opted-in subscribers during warm-up.
Prioritize engaging content: Send welcome emails, password resets, or other expected messages first.
Follow a gradual schedule: Incrementally increase sending volume based on engagement and deliverability metrics.
Monitor actively: Keep a close eye on Google Postmaster Tools, bounce rates, and spam complaints daily.
Strategies to mitigate and recover
When you encounter a high spam complaint rate during warm-up, swift action is necessary. First, pause or significantly reduce your sending volume. This gives you time to assess the situation without causing further damage to your sender reputation, which is critical for improving email deliverability.
Next, conduct a thorough audit of your email list. Identify and remove any unengaged subscribers, hard bounces, and especially any known spam traps. Implementing a double opt-in process for new subscribers can prevent similar issues in the future. Simultaneously, review your email content. Ensure your messages are clear, valuable, and consistent with what recipients expect. Avoid using spam-triggering words or overly promotional language during warm-up. If your transactional email spam rate is high, you should troubleshoot it immediately.
Finally, re-evaluate your warm-up schedule. It may be necessary to slow down your sending ramp-up significantly and focus on very small volumes of highly engaged users. Closely monitor your Google Postmaster Tools data daily and adjust your sending based on feedback, including open rates, click-through rates, and, of course, spam complaints. Remember, a successful warm-up is about building trust, not just sending volume.
Action plan for high spam rates
Immediate pause or reduction: Stop or significantly decrease sending volume to prevent further damage.
List re-evaluation: Segment your list, remove unengaged users, and verify addresses. For severe cases, consider how to re-warm a low volume domain.
Content optimization: Review content for relevance and tone, focusing on value to the recipient.
Monitor and adjust: Continuously track your metrics and be prepared to adapt your strategy. If your emails are going to spam, consider using an email deliverability tester.
Navigating your warm-up challenges
A high spam complaint rate during email warm-up, while concerning, is often a symptom of underlying issues rather than a death knell for your email program. It highlights the sensitivity of establishing a new sending reputation and the critical importance of meticulous planning and monitoring.
By understanding how mailbox providers calculate these rates, particularly the impact of low initial volumes, you can approach these spikes with a more informed perspective. Crucially, focusing on list hygiene, content relevance, and adhering to a disciplined warm-up schedule will set you up for long-term success.
Remember, the goal of warm-up is to build trust. When faced with high spam complaints, see it as an opportunity to refine your strategy, ensure you are sending to an engaged audience, and deliver content they genuinely want to receive. This proactive approach will help you navigate the warm-up period effectively and secure a strong sender reputation.
Views from the trenches
Best practices
Always use highly engaged and opted-in email lists for your warm-up process to minimize complaints and build trust.
Ensure your email content is highly relevant and expected by recipients during warm-up to encourage positive engagement.
Gradually increase your sending volume and frequency according to a structured warm-up schedule.
Monitor your spam complaint rates and other deliverability metrics daily using tools like Google Postmaster Tools.
Common pitfalls
Using old, unverified, or purchased email lists that can lead to hitting spam traps and high complaint rates.
Sending overly promotional or unexpected content during the sensitive warm-up phase, triggering spam reports.
Attempting to scale email volume too quickly, which is seen as suspicious by mailbox providers.
Neglecting to monitor deliverability metrics, allowing issues like high spam rates to go unaddressed and worsen.
Expert tips
Even small complaint numbers can inflate rates during low-volume warm-up due to how Google calculates the denominator.
Mailbox providers might be more forgiving of early spikes, but sustained high rates will still lead to trouble.
Complaints are attributed to the day an email was sent, not when the report is made by the user.
Spam rates in Postmaster Tools typically do not change retrospectively once reported for a specific day.
Marketer view
Marketer from Email Geeks says a couple complaints on a previous day’s email, and none of the emails inboxed the day the complaints were launched, or only a very small number, can lead to extremely high rates because only emails landing in the inbox are counted in the denominator.
2024-10-16 - Email Geeks
Marketer view
Marketer from Email Geeks says that Google's denominator for spam complaints is based on "active users," and even if a message inboxes, it doesn't mean the user is active.
Gmail and 
Outlook to recognize your sending patterns as legitimate, preventing your emails from being flagged as spam. It is a necessary step to build sender reputation and ensure high deliverability rates.
Google, calculate this rate based on spam complaints from active users for inboxed emails. This specific definition is critical, especially during the warm-up phase.
