A spam filter is a program designed to protect users from spam by filtering incoming emails. It helps prevent unwanted, unsolicited, or harmful content from reaching their inboxes.

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In this article, we’ll unveil the main types of spam filters and show how they work. We’ll also uncover why spam filters are important for email senders and provide you with tips for bypassing spam filters.

How do spam filters work?

To send the right emails to subscribers' inboxes, you need to understand how spam filters work. They analyze email messages to determine whether they are legitimate or look like spam. They apply various principles to evaluate the content, information about the sender, and other factors that influence email quality. Below, you can find more details on what spam filters monitor.

  • Content analysis. Spam filters analyze each email to identify suspicious patterns. It scans the text for spam-related words such as “free money," "limited time offer," and "urgent response required.” When the text doesn’t include such words, they examine the content for formatting specific for spammers. An excessive number of capital letters, symbols, or poor grammar are the signs for spam filters to mark these emails as spam. Then, they proceed to analyze the balance between images and text. When an email has a lot of pictures but little text, it looks suspicious. Finally, spam check attachments for malicious files are usually used for phishing schemes.
  • Header and metadata inspection. The next step is analyzing technical information in email, including the sender’s domain and IP address. By verifying these details, spam filters focus on detecting spoofing and blacklisted sources. Afterward, they check whether the emails were sent at odd hours or in bulk.
  • Sender’s reputation. Spam filters also pay attention to the reputation of the sender to find out whether the emails they send are legitimate. It involves checking whether a specific IP address or domain is on the blacklist or has a history of sending spam emails. The frequency of sending emails also matters. With filters, email providers can determine whether messages come from individuals engaging in fraudulent actions. When recipients report email senders as spam, it also encourages filters to block these senders.
  • Authentication checks. Emails should pass through authentication protocols to verify that they are legitimate and come from specified domains. While SPF ensures that emails are delivered from verified servers, DKIM controls the integrity of email content and prevents any sort of modification during transmission.
  • User interaction. User feedback allows spam filters to identify improper messages and prevents similar content from being delivered to recipients. By reporting spam, users help email providers significantly and contribute to sending only reliable and legitimate emails from verified sender addresses.
  • Scoring. Each email is given a specific score based on its elements and characteristics. A spam score is assigned after several factors are evaluated. These include suspicious keywords, sender reputation, and authentication checks. When an email receives a score that exceeds acceptable standards, it’s flagged as spam and isn’t delivered to the intended recipients.

Now that you know how spam filters work, it’s time to uncover what types of spam filters you can come across when sending email campaigns to your subscribers.

Types of Spam Filters

There are different types of spam filters depending on the areas they focus on when detecting unsolicited emails. To reach subscribers’ inboxes with high-quality emails, you need to understand the most popular types of filters you may encounter.

  • Content-based spam filters. Such filters focus on analyzing the content each email contains. They are usually checked for elements that are typical of spam. These include suspicious subject lines, attachments, formatting, phrases, symbols, or keywords that spammers usually use to attract users' attention. After that, each email is assigned a specific score based on how spammy it looks. The more spam elements the email contains, the lower the likelihood of reaching inboxes. The use of many capital letters, an excessive number of symbols, and keywords that encourage users to take immediate action, like “urgent action required” or “win now,” might become reasons for flagging emails as spam.
  • Header-based spam filters. This type of filter examines email headers and metadata for malicious actions. It checks the sender’s IP address, email routing path, and timestamp, helping to determine whether messages come from blacklisted IP addresses. When emails are sent at odd times or from different servers, they trigger spam filters and prevent these emails from reaching recipients’ inboxes. These filters are perfect for determining spoofed emails and preventing phishing attempts.
  • Rule-based spam filters. They operate based on predefined rules that help identify spam emails. These filters can reject emails with too many recipients or suspicious attachments. Companies usually customize them to meet their business needs but need to update them regularly for better performance and effectiveness.
  • Reputation-based spam filters. This type of spam filter assesses the sender’s domain, IP address, and email behavior to find out how reliable and legitimate their emails are. They scan blacklists to identify whether specific IPs or domains are included. Additionally, filters examine the sender's email activity history and how users react to these messages. When a sender has a poor reputation, filters block these emails and never deliver them to intended recipients. If unknown senders send emails in bulk, they can be flagged, too. This type of filter helps identify spammers and prevent repeat offenders from deceiving users.
  • Challenge-response spam filters. These filters are aimed at checking the identity of unknown senders and verifying they can be trusted. Once spam filters receive these emails, they send a verification message or a CAPTCHA. When they complete this challenge, their emails can be sent to recipients. If emails aren’t verified, they are automatically blocked by spam filters. They help identify automated spam and prevent it from getting into inboxes.
  • Hybrid spam filters. Such filters are a combination of different methods to analyze emails on different levels. By focusing on content, reputation, header, and other aspects simultaneously, these spam filters can ensure comprehensive protection from malicious emails aimed at fraud.

Now that you know the most common types of spam filters, let’s proceed to find out why they are essential for email senders.

Why are spam filters important for email senders?

Spam filters are crucial for both recipients and senders. When you have a business, it’s a must to maintain a positive reputation and ensure high email deliverability. Spam filters help you with these and other aspects of your business. Here are some more things spam filters do.

  • Increase email deliverability. When spam filters check your emails and assess their quality, they help you get into recipients’ inboxes. Using improper formatting, keywords, attachments, and other elements might cause your messages to land in subscribers’ spam folders and never be seen by recipients. Often getting into spam might harm your sender’s reputation and make it even more challenging to communicate with subscribers via email. However, if you evaluate your email campaigns before sending them, you can improve your email deliverability. The better your email campaigns look, the higher your chances of being noticed and receiving better engagement.
  • Prevent blacklisting. Spam filters are designed to identify suspicious emails and attempts to send bulk unsolicited emails. When they detect that senders have such elements in their emails, these messages get added to blacklists. This, in turn, negatively affects the sender's ability to deliver emails to intended recipients. As a result, businesses might face problems with communication and conversions. However, when you know how spam filters work and which elements are prohibited, you can create quality emails that reach your target audience. By following these requirements, your company effectively delivers the necessary messages and encourages the desired actions.
  • Comply with regulations. Spam filters often act based on anti-spam laws, including the CAN-SPAM Act, GDPR, and CASL. These regulations, which differ according to the region, help determine which emails are high-quality and which of them should be rejected. When you stick to the rules and regulations, you avoid various fines that come along with sending spam messages.
  • Boost engagement rates. By filtering spam emails, email providers ensure that only relevant and legitimate content lands in recipients’ inboxes. Meeting spam filter guidelines allows you to prevent emails from being marked as spam and effectively reach subscribers’ inboxes. This, in turn, enables you to increase engagement, including open and click-through rates. When subscribers interact with your emails, it positively impacts your sender's credibility.
  • Protect the sender's reputation. By creating emails that comply with regulations and laws, you avoid triggering spam filters and landing in spam folders. It also negatively affects the sender’s reputation based on IP and domain. A poor reputation harms your messages because they can’t reach recipients’ inboxes. However, when you send emails that contain all the necessary elements and relevant information, you can maintain a positive reputation.

Now that you have some reasons to consider spam filters for your business, it’s time to explore the tips. They will help you pass spam filters and reach the inboxes of intended recipients.

8 Tips for Bypassing Spam Filters and Landing in the Inbox

If you want to bypass spam filters and get into subscribers’ inboxes, you should get ready with a well-structured and logical approach. This should cover the most effective best practices related to email marketing. We’ll give you tips to ensure your emails catch recipients’ eye.

  1. Clean up your mailing lists. Checking your email lists for invalid or inactive addresses is a must for a positive sender’s reputation. When you get new subscribers, it’s better to use a double opt-in to confirm that the email addresses are valid and users are willing to receive your content. Also, consider segmenting your subscribers based on their behavior, needs, and problems. Then, you can personalize your interactions and cater to subscribers’ needs.
  2. Authenticate your emails. Email authentication protocols allow you to sign emails and verify their authenticity, protecting users from phishing attacks and malicious content. Depending on your objectives, consider setting up SPF, DKIM, and DMARC. These will help you appear reliable and legitimate.
  3. Send relevant and quality content. The email body, keywords, formatting, symbols, and other elements of your emails mean a lot when reaching recipients’ inboxes. Avoid using phrases like "free," "limited time offer," "win now,” and too many symbols because they trigger spam filters. Additionally, ensure that your emails look natural in terms of text-to-image ratio. Your emails should not only contain images but also valuable text. When writing subject lines for your email campaigns, ensure they don’t include misleading or clickbait phrases.
  4. Establish a positive sender’s reputation. A good sender’s reputation contributes to good email deliverability and better engagement. When you send email campaigns consistently based on the schedule and share valuable content that meets expectations, email providers positively evaluate your performance. Besides having positive sending behavior, you should monitor your bounce rates. This allows you to prevent problems from harming your reputation and solve issues before they arise.
  5. Craft emails based on subscribers’ needs. Your email content should align with your subscribers' expectations and address their pain points. This way, more subscribers will appreciate your emails and submit fewer spam complaints. Personalization allows you to connect with each recipient by tailoring messages to their specific needs. Leveraging data like previous purchases, browsing history, and shopping cart items can help you craft highly relevant, personalized email campaigns.
  6. Use a reliable email provider. By choosing a reputable email service provider, you can take advantage of their built-in features. These include analytics, spam testing, and deliverability tools.
  7. Check your IP. Consider constantly checking whether your IP is not on blacklists of suspicious senders. For this purpose, you can use platforms like MXToolbox or SenderScore. In case you use a new IP address, it is necessary to send more email campaigns to establish trust.
  8. Evaluate your campaign performance. Finally, constantly monitor metrics related to your email campaigns’ performance. It will help you promptly identify problems and solve them. That way, you’ll be able to avoid any harm to your sender's reputation and email deliverability.

To sum up, spam filters are essential for ensuring the proper quality of email campaigns for recipients while encouraging more interactions with your business. With an effective strategy and valuable tips, you can deliver emails that drive engagement, including open and click-through rates.

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