Why am I suddenly getting an influx of spam complaints?
Matthew Whittaker
Co-founder & CTO, Suped
Published 13 Jul 2025
Updated 19 Aug 2025
7 min read
Seeing a sudden influx of spam complaints can be alarming. One day your email program is cruising along with excellent sender reputation, and the next you're hit with a noticeable spike in complaints. This kind of sudden shift naturally makes you wonder what went wrong, especially when your email content or sending practices haven't changed.
It can feel like a deliverability mystery, particularly when diagnostic tools give conflicting information or don't pinpoint a clear cause. I've seen many senders face this, where previously low spam rates inexplicably jump. The immediate reaction is often to scrutinize every detail of your recent sends.
This situation is more common than you might think, and it rarely points to a single, obvious flaw. Often, it's a combination of subtle shifts in recipient behavior, evolving spam filtering algorithms, or even external factors beyond your direct control. Understanding these nuances is key to diagnosing and resolving the issue effectively.
Investigating the metrics of spam complaints
When you notice a sudden rise in spam complaints, the first step is to understand what's actually being measured. For instance, if you're tracking a Bulk Complaint Level (BCL) score from Microsoft Exchange Online Protection (EOP), it's important to know that this is an internal score used primarily for O365 users. O365 tenants set their own thresholds for what BCL score mail they will allow.
A BCL score of 5, for example, isn't necessarily a critical issue, even if it's higher than your usual. Many legitimate emails can receive this score, and only the most aggressive anti-spam administrators will block mail at this level. Crucially, BCL scores are distinct from the feedback loops (FBLs) that provide direct spam complaint data. Microsoft (outlook.com, hotmail.com) consumer email services do not use BCL scores directly for filtering and also do not provide FBLs, so your complaints are likely coming from other sources.
When you observe a sudden jump, especially if it coincides with a BCL score increase, it might indicate that your message appeared spammy to a larger segment of your audience, rather than a deep, underlying technical problem. This could be due to content, timing, or external factors that make your email stand out negatively during a specific period.
Understanding BCL scores
The Bulk Complaint Level (BCL) is a Microsoft Exchange Online Protection classification that tells you how likely an incoming email is to be considered bulk mail. Higher BCL values indicate the message is more likely to be considered spam. This score is internally generated and not directly tied to user spam complaints from services like Outlook.com or Hotmail. If you're seeing a spike in Outlook or Hotmail spam rates, the cause is usually elsewhere.
Common triggers for spam complaints
Beyond a single message's content, sudden influxes of spam complaints often point to shifts in your audience's perception or your list management. Even if your list is composed of known users, factors like unexpected email frequency or irrelevant content can trigger complaints. Subscribers may simply forget they opted in, especially if the last interaction was a while ago.
Another common cause is illegitimate subscriptions or bot sign-ups. If spambots add fake or old email addresses to your list, emails sent to these addresses can turn into spam traps or generate complaints from unsuspecting recipients. This can significantly damage your sender reputation, leading to higher blocklist (or blacklist) rates and poorer inbox placement. It's crucial to protect your lists from such attacks.
Sometimes, your emails might have previously landed in the spam folder, and a slight shift in a filter's sensitivity could make users more aware of them, prompting them to hit the 'This is Spam' button. This is especially true for transactional emails or marketing messages that might not be explicitly expected, even by legitimate users. Any mismatch between recipient expectation and email content can increase complaint rates.
Audience expectations
Ensure your content aligns with what subscribers expect based on how they opted in. Misaligned expectations are a primary driver of complaints, regardless of technical setup. For instance, if users signed up for product updates and receive a general marketing newsletter, they might mark it as spam.
Understanding root causes and technical issues
While content and audience engagement are critical, technical factors can also contribute to a sudden rise in spam complaints. Incomplete or incorrectly configured email authentication protocols like SPF, DKIM, and DMARC can severely impact your sender reputation. Even minor misconfigurations can cause emails to be flagged as suspicious by spam filters.
A compromised sending IP or domain can quickly lead to blocklisting. If your IP address or domain gets added to a major blocklist (or blacklist), your emails will likely be rejected or routed directly to spam folders, leading to more user complaints. This often happens if an account is hacked and used to send malicious mail. You should check your blocklist status regularly.
Content issues, such as suspicious links, excessive images, or spammy keywords, can also trigger filters. Even if your emails previously performed well, spam filter algorithms are constantly evolving. What worked last month might be flagged this month. It's a continuous balancing act to maintain compliance with email best practices.
Problem: high spam complaints
Sudden Spike: Complaint rate jumps from standard low levels to unusually high percentages overnight.
Inbox Placement: Emails that typically landed in the inbox are now being routed to spam folders, triggering manual complaints from users.
Audience Reaction: Subscribers mark emails as spam even if they are legitimate, indicating a disconnect in expectations or recognition.
Solution: proactive email management
List Hygiene: Implement double opt-in and regularly clean your email lists to remove inactive or problematic addresses.
Content Review: Audit your email content for potential spam triggers, including subject lines, links, and overall message structure. Understand why your emails go to spam.
Beyond the tests: the reality of deliverability
The world of deliverability tests can sometimes offer a false sense of security. Many of these tests rely on open-source software, which spammers also have access to. The proprietary algorithms used by major mailbox providers are not publicly disclosed. This means a 'passed' test doesn't guarantee inbox delivery, especially when actual recipient behavior isn't accounted for.
Much of what determines whether an email reaches the inbox is directly tied to how recipients interact with your mail. If users frequently delete your emails without opening them, or if they move them to spam, these actions send strong signals to filters. These behavioral metrics are rarely, if ever, measured by standard deliverability tests, making them less reliable for diagnosing sudden spikes in complaints.
To truly understand and mitigate sudden complaint spikes, you need to look beyond simple tests and focus on proactive monitoring. This includes analyzing your engagement metrics, monitoring feedback loops (where available), and carefully reviewing your sending practices for any subtle changes that could affect recipient perception. Regular checks of your domain reputation in tools like Google Postmaster Tools are also essential.
Views from the trenches
Best practices
Maintain a clean email list by regularly removing inactive or unengaged subscribers to prevent spam complaints and improve deliverability.
Implement double opt-in for all new subscribers to ensure explicit permission and reduce the likelihood of illegitimate sign-ups.
Monitor your sender reputation metrics, including spam complaint rates and blocklist (blacklist) status, in various postmaster tools.
Continuously review your email content for potential spam triggers, such as suspicious links or overused promotional language.
Common pitfalls
Relying solely on public deliverability tests, as they do not accurately reflect how major mailbox providers filter emails.
Ignoring subtle shifts in recipient engagement, which can signal underlying issues before they escalate into major spam complaints.
Sending emails to old or unverified lists, increasing the risk of hitting spam traps or generating complaints from unexpected recipients.
Overlooking seasonal trends or external events (like phishing scams) that might make legitimate emails appear suspicious.
Expert tips
Sender reputation is paramount. Focus on consistent, legitimate sending to a highly engaged audience.
Don't just look at spam complaint rates; analyze open rates, click-through rates, and unsubscribe rates for a holistic view.
Segment your audience and tailor content to specific interests to improve relevance and reduce the chance of complaints.
If complaints spike, review recent changes in email content, sending volume, or audience targeting first.
Expert view
Expert from Email Geeks says a sudden influx of spam complaints often suggests a spike in illegitimate subscriptions, where people receive mail they didn't ask for and report it as spam, or that mail was previously going to the spam folder and a recent message triggered complaints.
2022-09-02 - Email Geeks
Expert view
Expert from Email Geeks says the Bulk Complaint Level (BCL) score is an internal Microsoft score exclusively for O365 users, allowing tenants to choose which levels of BCL mail they accept into their organization.
2022-09-02 - Email Geeks
Keeping your inbox reputation strong
A sudden influx of spam complaints is a serious indicator that needs attention, but it's rarely a sign of an unsolvable problem. It often comes down to a combination of factors, including content relevance, recipient engagement, list hygiene, and technical configurations.
Prioritizing a clean list, maintaining strong sender reputation, and continuously aligning your email content with subscriber expectations are foundational to long-term deliverability success. By proactively monitoring these areas, you can quickly identify and address issues before they escalate, ensuring your messages continue to reach the inbox.
Microsoft Exchange Online Protection (EOP), it's important to know that this is an internal score used primarily for O365 users. O365 tenants set their own thresholds for what BCL score mail they will allow.
Outlook.com or 
Hotmail. If you're seeing a spike in Outlook or Hotmail spam rates, the cause is usually elsewhere.
