---
title: "Understanding Spam Filters"
description: "Learn how email spam filters work and how to ensure your messages reliably reach your contacts' inboxes."
url: https://help.user.com/en/articles/266-understanding-spam-filters
collection: "Understand Deliverability"
updated: 2026-10-28
language: en
alternates: [pl, fr, de, it, es]
---

# Understanding Spam Filters

Learn how email spam filters work and how to ensure your messages reliably reach your contacts' inboxes.

Email spam filters protect inboxes by reviewing incoming messages and stopping unwanted or unsafe content. Every time you send an email campaign, mailbox providers evaluate your message against specific rules to decide whether it belongs in the main inbox or the spam folder.

Understanding how spam filters evaluate your emails helps your team follow best practices, protect your sending reputation, and make sure your messages actually reach your audience.

## Definition

**Spam filters** are automated security systems that score incoming emails based on details like domain setup, sender reputation, message content, and list health. If a message receives a spam score higher than a provider's limit, the provider moves it to the spam folder or blocks it entirely.

Their primary job is to protect users from unwanted, harmful, or irrelevant messages (spam) by deciding whether an email should be delivered to the main Inbox or sent straight to the Spam folder.

## Key Evaluation Criteria

Spam filters look at several specific factors when scoring your emails:

-   **Domain Authentication**: Providers verify if your domain uses proper security settings like DKIM. Sending from a verified private domain builds trust, while free or unverified domains get flagged more easily.
    
-   **IP Reputation**: Email servers review the sending history of your IP address. If an IP address sends spam, deliverability drops for everyone sharing that address.
    
-   **Content and Format**: Filters check for messy code, excessive capital letters, exclamation marks, red text, or suspicious links. They also look at your image-to-text balance. The aim is to keep graphics under 40% of your email content.
    
-   **Contact List Quality:** Sending to non-existent addresses or contacts who mark your email as junk damages your reputation. [Clean lists](https://help.user.com/en/articles/155-how-to-clean-your-contact-list.md) with real opt-in contacts keep your scores healthy.
    

## How to Stay Out of Spam

To protect your deliverability and keep your emails in your recipients' inboxes, follow these practical steps:

-   Send emails from [your own domain](https://help.user.com/en/articles/51-how-to-connect-a-company-email-domain.md) that matches your website, rather than a free email address.
    
-   Set up DKIM domain authentication so inbox providers know your messages are legitimate.
    
-   Use [double opt-in signup](https://help.user.com/en/articles/254-how-to-implement-a-double-opt-in-process.md) forms so contacts confirm their address before receiving marketing emails.
    
-   Keep your copy short, clear, and focused without using misleading [subject lines](https://help.user.com/en/articles/264-tips-for-a-good-subject-line-for-better-email-deliverability.md) or aggressive trigger phrases.
    
-   [Test your email layouts](https://help.user.com/en/articles/143-how-to-test-email-messages.md) using “preview” option in the email builder and [A/B testing](https://help.user.com/en/articles/99-how-to-ab-test-email-campaigns.md) before sending.
    
-   Review your campaign performance to check open rates, bounces, and complaint reports.
    
-   Make [unsubscribing](https://help.user.com/en/articles/74-how-to-add-an-unsubscribe-link-to-the-email-message.md) easy. If you hide the unsubscribe link, frustrated users will hit the "Mark as spam" button instead, which severely damages your sender reputation.
    

> **Warning:** Never buy or rent email lists. Purchased lists contain invalid addresses, unengaged contacts, and spam traps that severely damage your domain reputation and cause providers to block your messages.

## Related Articles

-   [Understand Hard and Soft Bounces](https://help.user.com/en/articles/156-understand-hard-and-soft-bounces.md)
    
-   [Why Emails Go to the Spam Folder](https://help.user.com/en/articles/154-why-emails-go-to-the-spam-folder.md)