Why do emails sometimes end up in the spam folder?
Matthew Whittaker
Co-founder & CTO, Suped
Published 10 May 2025
Updated 17 Aug 2025
7 min read
It can be incredibly frustrating when emails you send, especially legitimate ones, consistently land in the spam or junk folder. You've crafted the perfect message, perhaps even used a reliable email service provider, yet your recipients aren't seeing your messages in their primary inbox. This is a common challenge that many senders face, from individual users to large businesses.
Email service providers (ESPs) like Gmail, Outlook, and Yahoo employ sophisticated spam filters designed to protect users from unwanted or malicious mail. These filters analyze numerous factors to determine if an incoming message is legitimate or if it should be flagged as spam.
Understanding these factors is crucial for improving your email deliverability and ensuring your messages reach their intended destination. We'll explore the primary reasons why emails end up in the spam folder and what you can do to prevent it.
The impact of sender reputation
Your sender reputation is arguably the most significant factor influencing where your emails land. This reputation is a score assigned to your sending IP address and domain, reflecting your trustworthiness as a sender. It's built over time based on your sending history, recipient engagement, and compliance with email best practices.
A poor sender reputation can lead to your emails being filtered into spam folders, or even blocked entirely. Factors that negatively impact your reputation include high bounce rates, low open and click-through rates, and, most critically, recipients marking your emails as spam. This is why list hygiene and sending only to engaged subscribers are paramount.
ISPs continuously monitor sender behavior. If they detect patterns associated with spam, such as sending to unknown users or a sudden spike in volume, your reputation can suffer quickly. Maintaining a consistently good reputation requires ongoing vigilance and adherence to email marketing best practices.
Best practices for sender reputation
List hygiene: Regularly remove inactive or unengaged subscribers from your lists.
Gradual sending: Warm up new IP addresses or domains by gradually increasing sending volume.
Monitor performance: Use tools like Google Postmaster Tools to track your sender reputation metrics.
Clear unsubscribe: Always include an easy-to-find unsubscribe link.
Authentication failures (SPF, DKIM, DMARC)
Email authentication protocols, namely Sender Policy Framework (SPF), DomainKeys Identified Mail (DKIM), and Domain-based Message Authentication, Reporting, and Conformance (DMARC), are fundamental for proving that your emails are legitimate. When these records are missing or misconfigured, ISPs become highly suspicious of your messages.
SPF specifies which mail servers are authorized to send email on behalf of your domain. DKIM adds a digital signature to your outgoing emails, allowing the recipient's server to verify that the email hasn't been tampered with. DMARC builds upon SPF and DKIM, providing instructions to receiving servers on how to handle emails that fail authentication, such as quarantining or rejecting them. Failing these checks can directly lead to emails ending up in the spam folder. To understand more about the differences between these, see a simple guide to DMARC, SPF, and DKIM.
Implementing and correctly configuring these records is not optional anymore; it's a requirement for good deliverability, especially with new sender requirements from major mailbox providers. Many issues can arise, such as DMARC verification failed errors. If you are trying to understand your reports more, we recommend looking at this guide on understanding and troubleshooting DMARC reports. You can also use a free DMARC record generator tool.
Even with perfect technical setup, your email content itself can trigger spam filters. ISPs analyze the subject line, body text, image-to-text ratio, and even the number of links to identify potential spam. Certain words or phrases, often called spam trigger words, can increase the likelihood of your email being flagged.
Beyond content, recipient engagement plays a huge role. If your emails consistently have low open rates, low click-through rates, or high unsubscribe rates, it signals to ISPs that your content might not be relevant or desired. This negative engagement can harm your sender reputation, making it more likely that future emails will go to spam.
Poor email design, excessive use of all caps, too many exclamation marks, or misleading subject lines can also lead to spam folder placement. Recipients are more likely to mark such emails as spam, further damaging your reputation. Prioritizing clear, concise, and valuable content that resonates with your audience is key. For more tips, check out this resource on why emails go to spam.
Good content practices
Relevant content: Deliver value that matches subscriber expectations.
Engaging subject lines: Be clear and concise, avoiding clickbait.
Balanced design: Maintain a good text-to-image ratio and proper formatting.
Personalization: Segment your audience and tailor content for better engagement.
Spam content triggers
Spammy keywords: Words like 'free money,' 'winner,' 'guarantee,' and excessive capitalization.
Poor formatting: Broken HTML, large fonts, unusual colors.
Excessive links/images: Overloading emails with too many hyperlinks or large image files.
Deceptive subject lines: Misleading recipients about the email's content.
Recipient actions and blocklist impact
Ultimately, if a recipient marks your email as spam, it sends a strong negative signal to their email provider. This action directly impacts your sender reputation and can lead to future emails being filtered more aggressively. Even if an email is legitimate, a user who doesn't remember subscribing or finds the content irrelevant might click 'Mark as Spam' rather than unsubscribing.
Furthermore, if your sending IP address or domain is listed on an email blocklist (or blacklist), it means you've been identified as a source of spam or other undesirable email. Blocklists are used by many ISPs to filter out suspected spam before it even reaches a user's inbox. Getting on one of these lists can severely cripple your email deliverability, pushing all your emails to spam or causing them to be rejected outright.
There are hundreds of public and private blocklists, each with its own criteria for listing and de-listing. Some are highly influential, like Spamhaus, while others are less impactful. Regardless, if you find yourself on a blocklist, it's crucial to identify the cause, rectify the issue, and then request removal. We have a detailed guide to email blocklists to help you understand them.
Even for legitimate senders, email deliverability is an ongoing process of monitoring and optimization. A single user marking an email as spam can send a strong signal to mailbox providers that affects your reputation. If you're experiencing issues with emails landing in spam, checking for blocklist presence is an essential diagnostic step. Our blocklist checker can help you determine if your domain or IP is listed.
Understanding email blocklists (blacklists)
Purpose: Blocklists help ISPs filter unwanted email by identifying suspicious senders.
Listing triggers: High spam complaints, sending to spam traps, or malicious activity.
Impact: Emails may be rejected or routed to the spam folder, regardless of content.
Always use double opt-in for new subscribers to confirm their intent and prevent spam trap issues.
Regularly clean your email lists to remove inactive users and reduce bounce rates, which can improve sender reputation.
Implement strong email authentication (SPF, DKIM, DMARC) to prove your legitimacy to receiving servers and improve deliverability.
Common pitfalls
Ignoring email authentication, which makes your emails appear suspicious to spam filters and significantly impacts deliverability.
Sending emails to purchased or old lists without proper hygiene, leading to high bounce rates and spam trap hits.
Using deceptive subject lines or excessive 'spammy' keywords, triggering content-based filters.
Expert tips
A Microsoft expert says that even within their own ecosystem, Microsoft email sometimes lands in the spam folder, emphasizing that no sender is exempt from strict filtering rules.
An expert highlights the need for senders to follow best practices consistently.
An expert notes that even if an email service provider is part of the same company, it does not mean their emails get a pass from spam filtering.
Expert view
Expert from Email Geeks says even internal tools can sometimes miscategorize legitimate emails, highlighting the nuanced nature of spam filtering.
2023-10-26 - Email Geeks
Marketer view
Marketer from Email Geeks says end-users marking emails as spam significantly impacts sender reputation.
2023-10-26 - Email Geeks
Achieving inbox success
Email deliverability is a multifaceted challenge influenced by technical configurations, content quality, sender reputation, and recipient behavior. Emails don't just 'end up' in spam folders by chance; it's a result of a complex interplay of filtering rules and signals that ISPs use to protect their users.
To maximize your inbox placement, a holistic approach is necessary. This includes ensuring robust email authentication, maintaining a stellar sender reputation through consistent positive engagement, and crafting valuable content that avoids spam triggers. By proactively addressing these areas, you can significantly reduce the chances of your emails being misclassified.
Remember, email deliverability is an ongoing process. Regular monitoring and adaptation to evolving ISP rules are key to consistently reaching your audience's inbox. Prioritizing your recipients' experience and adhering to best practices will always be your best defense against the spam folder.
Gmail, 
Outlook, and 
Yahoo employ sophisticated spam filters designed to protect users from unwanted or malicious mail. These filters analyze numerous factors to determine if an incoming message is legitimate or if it should be flagged as spam.
Google Postmaster Tools to track your sender reputation metrics.
Spamhaus, while others are less impactful. Regardless, if you find yourself on a blocklist, it's crucial to identify the cause, rectify the issue, and then request removal. We have a detailed guide to email blocklists to help you understand them.
