How Gmail's Spam Filter Works
Rather than following a simple list of rules, the spam filter evaluates multiple signals simultaneously for every incoming email. According to Google's official blog, "Gmail employs a number of AI-driven filters that determine what gets marked as spam. These filters look at a variety of signals, including characteristics of the IP address, domains/subdomains, whether bulk senders are authenticated, and user input."
When an email arrives, Gmail checks whether the sender's domain has proper authentication configured (SPF, DKIM, and DMARC records). Messages from domains that fail these checks are far more likely to land in spam because Gmail cannot verify the sender is who they claim to be.
In addition to authentication, the filter analyzes the message itself for patterns commonly associated with spam, such as suspicious links, certain formatting tricks, or phrases frequently found in unwanted bulk mail.
But arguably the most powerful aspect of Gmail's spam filter is its ability to learn from user actions. Google explains that "user feedback, such as when a user marks a certain email as spam or signals they want a sender's emails in their inbox, is key to this filtering process, and our filters learn from user actions." So, when you mark an email as spam in Gmail, that feedback helps improve detection for everyone. The same applies when you rescue a legitimate email from your spam folder. And over 1.8 billion monthly active users globally do it, the spam filter becomes really, really capable.
The filter also considers your personal email history. Senders you regularly correspond with are less likely to be flagged, while addresses you have previously blocked or reported will have their future messages filtered automatically. Google states that the system blocks "more than 99.9 percent of spam, phishing, and malware from ever reaching users' inboxes."
How to Mark Emails as Spam or Not Spam
The two most important actions you can take to control Gmail's spam filter are reporting unwanted emails as spam and rescuing legitimate messages that were incorrectly filtered. As explained in the previous section, both actions directly train Gmail's algorithms to work better for you and for all users.
Reporting an Email as Spam
When an unwanted message lands in your inbox, select it (or open it) and click the Report spam button in the toolbar (looks like a stop sign with an exclamation point).
Gmail immediately moves the message to your Spam folder and sends a copy to Google for analysis. Future emails from that sender will typically be routed to Spam automatically.
💡 Tip: For newsletters or promotional emails you no longer want, you can use the Unsubscribe link that appears near the sender's address instead of reporting spam instead. Legitimate senders honor these requests, and unsubscribing is the cleaner solution for emails you once signed up for because it will prevent messages from trying to reach you in the first place.
Marking an Email as Not Spam (Whitelisting)
If you find a legitimate email in your Spam folder, open the Spam folder from the left menu (you may need to click More to see it), open the message, and click the Report not spam button at the top.
The action moves the email back to your inbox and tells Gmail to stop filtering messages from that sender. According to Google's support documentation, future emails from the same sender should no longer go to Spam once you mark one as legitimate.
If you want to be extra sure that you won't miss a message from someone important, you can add the sender to your Google Contacts. You can learn more about creating a Gmail safe sender list for senders you always want to receive.
How to Create Custom Filters in Gmail
Gmail's built-in spam detection handles most unwanted emails automatically, but custom filters give you precise control over specific messages, so you can use them to keep even "legitimate" senders out of your inbox. They can also be used to automatically sort, label, archive, or protect emails based on criteria you define.
Creating a New Filter
To create a filter, click the Show search options icon (the small slider icon) in Gmail's search bar at the top of the page.
A dialog box appears where you can specify your filter criteria. Gmail offers several options:
- From: Filter by sender's email address or domain (e.g., @company.com).
- To: Filter by recipient address (useful if you use multiple aliases).
- Subject: Match specific words in the subject line.
- Has the words: Match keywords anywhere in the email.
- Doesn't have: Exclude emails containing certain terms.
- Size: Filter by message size (greater or less than a specified amount).
- Has attachment: Target only emails with files attached.
After entering your criteria, click Create filter at the bottom. Gmail then asks what action to take when emails match your filter.
Filter Actions for Spam Control
Two actions are especially relevant for managing spam:
- Never send it to Spam: This option guarantees that matching emails always reach your inbox, regardless of what Gmail's spam filter thinks. For example, you might create a filter for emails from your bank's domain and select "Never send it to Spam" to guarantee delivery.
- Delete it: With this, you can effectively create a personal blocklist. If a particular sender or type of email keeps bothering you and Gmail does not catch it, you can filter those messages directly to Trash.
You can also apply labels, mark emails as read, forward them, or categorize them into Primary, Social, or Promotions tabs. Checking Also apply filter to matching conversations applies your new rule to existing emails, not just future ones.
For more detailed guidance on organizing your inbox with rules, see our guide on creating Gmail rules.
You can always review your filters by going to Settings (gear icon) → See all settings → Filters and Blocked Addresses.
Fixing Errors and Optimizing Your Filters
Gmail's spam filter occasionally makes mistakes and knowing how to correct them helps you maintain a reliable inbox where important messages always arrive and junk stays out.
False Positives
A false positive occurs when Gmail incorrectly marks a wanted email as spam. This might happen because the sender's domain lacks proper authentication, the message contains phrases commonly found in spam, or other users have reported similar emails.
To fix a false positive, locate the email in your Spam folder, select it, and click Not spam. This tells Gmail the message is legitimate and trains the filter to recognize similar emails in the future. For senders who regularly get flagged, create a filter with the "Never send it to Spam" action or add them to your contacts.
💡 Tip: If you notice certain types of emails consistently landing in spam, there may be an underlying issue worth investigating. Our guide on why emails go to spam covers common causes and solutions.
False Negatives
A false negative happens when unwanted junk slips past the filter and reaches your inbox (new spam tactics Gmail has not yet learned to recognize or messages that cleverly mimic legitimate emails).
When spam reaches your inbox, always use the Report spam button rather than simply deleting it. Reporting sends the email to Google for analysis, which improves detection for you and all Gmail users. Just deleting the message teaches Gmail nothing about your preferences.
💡 Tip: For persistent spam from the same source, you can block the sender on Gmail or create a filter to automatically delete their messages.
Best Practices for Keeping Gmail Filter Accurate
Gmail's spam filter works best when you actively participate in training it, and the following habits can help the system make less mistakes over time and save you a lot of headache:
- Check your Spam folder regularly. Gmail automatically deletes spam after 30 days, so any legitimate email trapped there will be lost if you don't act quickly enough.
- Use the spam and not spam buttons consistently. Every time you report spam or mark a message as legitimate, you provide feedback that helps Gmail learn. Skipping this step means missed opportunities to improve your filter's accuracy.
- Avoid over-filtering. When creating custom filters, use specific criteria rather than broad terms. A filter that deletes everything with "sale" in the subject line will catch promotional spam but also legitimate order confirmations.
- Add important contacts to your address book. Gmail is less likely to flag emails from addresses in your Google Contacts. For senders whose messages you absolutely cannot miss, adding them as contacts provides an extra layer of protection against false positives.
- Unsubscribe instead of reporting legitimate newsletters as spam. If you signed up for a mailing list but no longer want to receive it, use the unsubscribe option. Reporting legitimate bulk mail as spam can affect the sender's reputation unfairly and does not stop future emails as reliably as a proper unsubscribe request.
The few minutes you invest each day into putting these best practices into, well, practice, will pay off in a cleaner, more reliable inbox.
How Clean Email Can Enhance your Gmail Spam Filter
When Gmail's built-in spam filter isn’t enough to stop persistent spam or unknown senders, Screener by Clean Email acts as a powerful, third-party spam filter for Gmail. It helps stop junk emails at the source by automatically screening and filtering messages from unknown senders.
First-time senders are placed in a review folder, allowing you to decide whether to block or approve them. Approved senders will go to your inbox in the future, while blocked ones will be filtered out.
Try it for Free in your inbox!
Clean Email works alongside Gmail to handle the bulk cleanup and ongoing maintenance that Gmail's spam filter was not designed for. While Gmail focuses on catching malicious and unsolicited messages, Clean Email helps you manage the emails that technically belong in your inbox but contribute to overload.
Bulk Cleanup of Legitimate Clutter
Clean Email groups your emails by sender, type, and other criteria to make it easy to review and remove thousands of messages at once. Instead of deleting promotional emails one by one, you can clear entire categories in a few clicks.
Finding Large and Old Emails
Emails with large attachments and years-old messages can consume storage space and make it harder to find what matters. Clean Email lets you filter by size and age to identify these emails quickly and help you stop unwanted emails in Gmail from taking over your account.
Automating Ongoing Maintenance
The Auto Clean feature lets you create rules that run automatically on incoming mail. For example, you can set emails from certain senders to be archived after a week or have old promotional messages moved to trash automatically.
A cleaner inbox means Gmail's spam filter has less noise to compete with, and you can easily spot important messages and catch any filtering mistakes before they cause problems.
Conclusion
Gmail's spam filter is one of the most effective email protection systems available thanks to the combination of machine learning, authentication checks, and user feedback, which allows the system to adapt to new threats while learning your personal preferences. However, no automated filter is perfect. The techniques covered in this guide give you direct control over how Gmail handles your email.
