How to Check Spam Filters: A Practical Guide

Email deliverability is crucial for any online business or communication. One of the biggest hurdles is navigating spam filters, which are constantly evolving to combat unwanted messages. Understanding how these filters work and knowing how to check if your emails are being caught by them is vital. This guide will provide practical methods for testing your email deliverability and identifying potential issues, focusing on specific techniques you can implement today to improve your email sending reputation.

Table of Contents

• Understanding Spam Filter Mechanisms
• Using Seed List Testing
• Leveraging Authentication Methods (SPF, DKIM, DMARC)
• Analyzing Feedback Loops and Bounce Messages
• Interpreting Spam Filter Results and Taking Action

Understanding Spam Filter Mechanisms

Spam filters employ a variety of techniques to identify and block unsolicited or malicious emails. Knowing these mechanisms is the first step in avoiding them. These techniques can be broadly categorized into content-based filters, reputation-based filters, and authentication-based filters.

Content-based filters analyze the content of an email for characteristics commonly found in spam. This includes keywords, phrases, links, and formatting. They assign scores based on these characteristics, and if the total score exceeds a certain threshold, the email is flagged as spam. For example, using excessive exclamation points or words like “free,” “guarantee,” or “urgent” can increase your spam score.

Reputation-based filters assess the sender’s reputation based on their IP address, domain, and sending history. If the IP address or domain is blacklisted due to previous spamming activities, emails from that source are likely to be blocked. Reputation is built over time by consistently sending legitimate emails and adhering to best practices. Tools like Sender Score (senderscore.org) can help you monitor your IP reputation.

Authentication-based filters verify the sender’s identity using protocols like SPF, DKIM, and DMARC. These protocols help prevent email spoofing and phishing by confirming that the sender is authorized to send emails on behalf of the domain. Without proper authentication, emails are more likely to be flagged as spam.

Content-Based Filter Examples

Here are some examples of how content can trigger spam filters:

• Excessive Use of Spam Trigger Words: Words like “Viagra,” “Lottery,” “Free,” “Guarantee,” “Urgent,” and excessive use of “!!!” can significantly increase your spam score.
• Poor HTML Formatting: Emails with broken HTML, overly large images, or an excessive number of fonts and colors are often flagged.
• Misleading Subject Lines: Subject lines that don’t accurately reflect the content of the email can also lead to spam filtering.
• Shortened URLs: Using URL shortening services excessively can make your emails look suspicious to filters.

Reputation-Based Filter Examples

Your sending reputation is determined by various factors:

• Blacklisting: Being listed on a real-time blacklist (RBL) like Spamhaus (spamhaus.org) will severely impact your deliverability. You can check if your IP is blacklisted using their lookup tool.
• Spam Traps: Sending emails to spam trap addresses (addresses created solely to identify spammers) can damage your reputation.
• High Bounce Rates: A high bounce rate indicates that you are sending emails to invalid or inactive addresses.
• Low Engagement Rates: Low open rates and click-through rates can signal to mailbox providers that your emails are not relevant or desired by recipients.

Authentication-Based Filter Examples

Proper email authentication is vital for establishing trust:

• SPF (Sender Policy Framework): SPF records specify which mail servers are authorized to send emails on behalf of your domain. A missing or misconfigured SPF record can lead to emails being rejected or flagged as spam.
• DKIM (DomainKeys Identified Mail): DKIM adds a digital signature to your emails, allowing receiving servers to verify that the email was sent by an authorized source and that the content hasn’t been altered in transit.
• DMARC (Domain-based Message Authentication, Reporting & Conformance): DMARC builds upon SPF and DKIM by providing instructions to receiving servers on how to handle emails that fail authentication checks. It also provides reporting mechanisms to help domain owners monitor their email authentication performance.

Example: Checking SPF Record

nslookup -type=txt yourdomain.com

# Example Output:
# "v=spf1 include:_spf.google.com ~all"

This command uses the nslookup tool to query the DNS records for your domain (yourdomain.com). The -type=txt option specifies that we are looking for TXT records, which is where SPF records are stored. The output shows the SPF record for the domain. The example above includes Google’s SPF record, allowing Google servers to send emails on behalf of the domain. The ~all mechanism indicates a soft fail policy, meaning that emails from servers not listed in the SPF record may still be accepted but are more likely to be flagged as spam.

Understanding these mechanisms is essential for developing effective strategies to improve your email deliverability and ensure that your messages reach their intended recipients.

Using Seed List Testing

Seed list testing is a valuable method for evaluating your email deliverability across various email providers before sending to your entire audience. It involves sending your email to a predetermined list of email addresses (the “seed list”) hosted by different mailbox providers like Gmail, Yahoo, Outlook, and others. By analyzing how these providers handle your email, you can identify potential deliverability issues and make necessary adjustments.

There are numerous seed list testing services available, such as Litmus, Email on Acid, GlockApps, and Mail-Tester. These services provide you with a unique set of email addresses to include in your email campaign. After sending your email, they analyze the results and provide detailed reports on your deliverability, including which mailbox providers are accepting your email, which are sending it to the spam folder, and any potential issues that may be impacting your deliverability.

Setting Up a Seed List Test

Here’s a general outline of how to conduct a seed list test:

• Choose a Seed List Testing Service: Select a service that meets your needs and budget. Consider factors like the number of email providers covered, the level of detail in the reports, and the features offered (e.g., spam filter analysis, authentication checks).
• Create a Seed List: The service will provide you with a list of email addresses (the seed list). These addresses are strategically distributed across different mailbox providers.
• Send Your Email: Include the seed list email addresses in the recipients of your email campaign. Ensure that the email is representative of the types of emails you typically send.
• Analyze the Results: After sending your email, the seed list testing service will analyze the results and provide you with a report. This report will typically include information on your deliverability rate, spam score, authentication status, and any potential issues that were identified.

Example Seed List Testing Service: Mail-Tester.com

Mail-Tester.com is a free and easy-to-use seed list testing service. It provides you with a unique email address to which you send your email. After sending, it analyzes your email and provides a detailed report on your spam score, authentication status, and content analysis.

Example Scenario: Using Mail-Tester

• Step 1: Visit Mail-Tester.com. You will be presented with a unique email address.
• Step 2: Send your email to the provided email address.
• Step 3: Click the “Then check your score” button on Mail-Tester.com.
• Step 4: Mail-Tester will analyze your email and display a score out of 10, along with detailed information on your email’s authentication status, content analysis, and any potential issues.

Mail-Tester can identify issues such as missing SPF or DKIM records, broken links, or content that triggers spam filters. It provides actionable recommendations for improving your email deliverability.

Interpreting Seed List Results

The key is to pay attention to the following:

• Deliverability Rate: The percentage of seed list emails that reached the inbox versus the spam folder or were blocked. A high deliverability rate is desirable.
• Spam Score: A score indicating how likely your email is to be flagged as spam. Lower scores are better.
• Authentication Status: Confirmation that SPF, DKIM, and DMARC are properly configured.
• Content Analysis: Identifies specific words, phrases, or formatting issues that may be triggering spam filters.

Example: Analyzing a Seed List Report

Metric	Value	Interpretation
Deliverability Rate	90%	Good, but could be improved. Investigate the 10% that didn’t reach the inbox.
Spam Score (out of 10)	6	Potentially problematic. Needs investigation.
SPF	Pass	SPF is configured correctly.
DKIM	Fail	DKIM is not configured or is failing. This is a critical issue.
Content Analysis	Identified 3 spam trigger words	Review the content and replace or remove the spam trigger words.

In this example, the report indicates that DKIM is failing and that the email contains spam trigger words. Addressing these issues is crucial for improving email deliverability.

By consistently using seed list testing, you can proactively identify and address deliverability issues, ensuring that your emails reach your intended recipients and maximizing the effectiveness of your email campaigns.

Leveraging Authentication Methods (SPF, DKIM, DMARC)

Email authentication is the cornerstone of establishing trust with mailbox providers and preventing your emails from being flagged as spam. Implementing SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting & Conformance) is essential for verifying your sender identity and ensuring that your emails are delivered to the inbox.

These protocols work together to provide a comprehensive email authentication system. SPF specifies which mail servers are authorized to send emails on behalf of your domain. DKIM adds a digital signature to your emails, allowing receiving servers to verify that the email was sent by an authorized source and that the content hasn’t been altered in transit. DMARC builds upon SPF and DKIM by providing instructions to receiving servers on how to handle emails that fail authentication checks and provides reporting mechanisms to help domain owners monitor their email authentication performance.

Implementing SPF

To implement SPF, you need to create a TXT record in your domain’s DNS settings. This record lists the mail servers that are authorized to send emails on behalf of your domain. Consult with your email provider (e.g., Google Workspace, Microsoft 365, SendGrid) to determine the correct SPF record to use.

Example: SPF Record

v=spf1 include:_spf.google.com include:sendgrid.net ~all

This SPF record indicates that Google Workspace and SendGrid are authorized to send emails on behalf of your domain. The ~all mechanism indicates a soft fail policy. You can use -all for a hard fail, meaning that emails from servers not listed in the SPF record should be rejected.

Example: Adding SPF Record via cPanel

• Step 1: Log in to your cPanel account.
• Step 2: Navigate to the “Zone Editor” section.
• Step 3: Locate your domain and click “Manage.”
• Step 4: Click “+ Add Record” and select “TXT” as the record type.
• Step 5: Enter your domain name in the “Name” field.
• Step 6: Enter the SPF record value (e.g., v=spf1 include:_spf.google.com ~all) in the “Record” field.
• Step 7: Click “Add Record.”

Implementing DKIM

Implementing DKIM involves generating a public/private key pair and adding the public key to your domain’s DNS settings. Your email server then uses the private key to sign outgoing emails, and receiving servers use the public key to verify the signature.

The exact steps for generating a DKIM key and adding it to your DNS settings will vary depending on your email provider. Refer to your email provider’s documentation for specific instructions.

Example: DKIM Record (TXT Record)

default._domainkey.yourdomain.com.  IN      TXT     "v=DKIM1; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA6Z163wq..."

This is an example of a DKIM record. The v=DKIM1 tag specifies the DKIM version, k=rsa specifies the key type, and p= contains the public key. The actual public key will be a long string of characters.

Implementing DMARC

DMARC builds upon SPF and DKIM by providing instructions to receiving servers on how to handle emails that fail authentication checks. It also provides reporting mechanisms to help domain owners monitor their email authentication performance.

To implement DMARC, you need to create a TXT record in your domain’s DNS settings named _dmarc.yourdomain.com. This record specifies the DMARC policy and reporting options.

Example: DMARC Record

_dmarc.yourdomain.com.  IN      TXT     "v=DMARC1; p=none; rua=mailto:dmarc-reports@yourdomain.com; ruf=mailto:dmarc-forensic@yourdomain.com; adkim=r; aspf=r;"

This DMARC record specifies the following:

• v=DMARC1: Specifies the DMARC version.
• p=none: Specifies the DMARC policy. In this case, the policy is set to “none,” meaning that receiving servers should take no action on emails that fail authentication checks. This is a good starting point for monitoring your DMARC implementation. You can later change the policy to “quarantine” (move emails to the spam folder) or “reject” (reject emails) once you are confident in your authentication setup.
• rua=mailto:dmarc-reports@yourdomain.com: Specifies the email address to which aggregate reports should be sent. These reports provide an overview of your email authentication performance.
• ruf=mailto:dmarc-forensic@yourdomain.com: Specifies the email address to which forensic reports (also known as failure reports) should be sent. These reports provide detailed information about individual emails that failed authentication checks.
• adkim=r: Specifies the alignment mode for DKIM. “r” means relaxed.
• aspf=r: Specifies the alignment mode for SPF. “r” means relaxed.

Expert Tip: Start with a DMARC policy of p=none and gradually move to p=quarantine and then p=reject as you gain confidence in your email authentication setup. Monitor the DMARC reports to identify and address any issues before implementing a more restrictive policy.

By implementing SPF, DKIM, and DMARC, you can significantly improve your email deliverability and protect your domain from email spoofing and phishing attacks. Regular monitoring of your DMARC reports is crucial for identifying and addressing any issues that may arise.

Analyzing Feedback Loops and Bounce Messages

Understanding and responding to feedback loops (FBLs) and bounce messages is critical for maintaining a healthy email sending reputation and improving deliverability. These mechanisms provide valuable information about how mailbox providers and recipients are interacting with your emails, allowing you to identify and address potential issues.

Feedback Loops (FBLs): FBLs are a mechanism by which mailbox providers notify senders when their emails are marked as spam by recipients. By subscribing to FBLs, you can receive reports containing information about which users are marking your emails as spam. This allows you to remove those users from your mailing list, preventing future spam complaints and improving your sending reputation.

Bounce Messages: Bounce messages are automated responses from mail servers indicating that an email could not be delivered. There are two main types of bounces: hard bounces and soft bounces. Hard bounces indicate a permanent delivery failure (e.g., invalid email address), while soft bounces indicate a temporary delivery failure (e.g., mailbox full). Properly handling bounce messages is essential for maintaining a clean mailing list and avoiding deliverability issues.

Setting Up Feedback Loops

To set up feedback loops, you need to register with each mailbox provider that offers them. The registration process typically involves verifying ownership of your sending IP address or domain.

Here are some of the major mailbox providers that offer feedback loops:

• Gmail: Google Postmaster Tools (postmaster.google.com) provides comprehensive insights into your email sending reputation and deliverability, including spam complaint rates.
• Microsoft (Outlook, Hotmail, Live): Microsoft’s Smart Network Data Services (SNDS) (sendersupport.olc.microsoft.com) provides data on your IP reputation and spam complaint rates. You can also use the Junk Email Reporting Program (JMRP) to receive reports when users mark your emails as junk.
• Yahoo: Yahoo’s Complaint Feedback Loop (CFL) allows you to receive reports when Yahoo Mail users mark your emails as spam.

Example: Setting Up Google Postmaster Tools

• Step 1: Visit Google Postmaster Tools (postmaster.google.com).
• Step 2: Sign in with your Google account.
• Step 3: Add your sending domain.
• Step 4: Verify ownership of your domain by adding a TXT record to your DNS settings. Google will provide you with the TXT record value.
• Step 5: Once your domain is verified, Postmaster Tools will provide you with data on your email sending reputation, spam complaint rates, and other deliverability metrics.

Handling Bounce Messages

Properly handling bounce messages is crucial for maintaining a clean mailing list and avoiding deliverability issues. You should automatically remove hard-bounced email addresses from your mailing list immediately. Soft bounces can be retried a few times, but if an address consistently soft bounces, it should also be removed.

Example: Bounce Message Analysis

Diagnostic-Code: smtp; 550 5.1.1 User unknown

This bounce message indicates a hard bounce due to an invalid email address (User unknown). This address should be immediately removed from your mailing list.

Diagnostic-Code: smtp; 450 4.2.1 Mailbox temporary disabled - quota exceeded

This bounce message indicates a soft bounce due to the recipient’s mailbox being full (quota exceeded). You can retry sending to this address a few times, but if it continues to bounce, it should be removed.

Analyzing FBL and Bounce Data

Regularly analyze your FBL and bounce data to identify and address potential issues. Look for trends in spam complaints and bounce rates. A sudden increase in spam complaints or bounce rates may indicate a problem with your email content, sending practices, or list hygiene.

Example: Analyzing Google Postmaster Tools Data

• Spam Rate: Monitor your spam rate in Postmaster Tools. A high spam rate indicates that a significant number of Gmail users are marking your emails as spam. This can negatively impact your sending reputation and deliverability.
• IP Reputation: Postmaster Tools provides data on your IP reputation. A “Bad” or “Low” IP reputation indicates that your IP address has been associated with spamming activities.
• Domain Reputation: Postmaster Tools also provides data on your domain reputation. A “Bad” or “Low” domain reputation indicates that your domain has been associated with spamming activities.

By actively monitoring and responding to feedback loops and bounce messages, you can maintain a clean mailing list, improve your sending reputation, and ensure that your emails reach their intended recipients.

Interpreting Spam Filter Results and Taking Action

After conducting various tests and analyzing data from feedback loops and bounce messages, the next crucial step is to interpret the results and take appropriate action to improve your email deliverability. This involves identifying the underlying causes of any deliverability issues and implementing strategies to address them.

The interpretation process requires a holistic view, combining insights from different sources. For example, a high spam score in a seed list test might correlate with a poor domain reputation in Google Postmaster Tools. This combination suggests that your domain is perceived as a source of spam and requires immediate attention.

Analyzing Spam Filter Reports

Spam filter reports from seed list testing services provide valuable insights into why your emails are being flagged as spam. These reports typically highlight specific issues related to your email content, authentication, and infrastructure.

Here’s how to interpret common findings in spam filter reports:

• High Spam Score: A high spam score indicates that your email contains characteristics that are commonly associated with spam. Review the content analysis section of the report to identify specific words, phrases, or formatting issues that are triggering spam filters.
• Authentication Failures: Authentication failures (e.g., SPF fail, DKIM fail, DMARC fail) indicate that your email is not properly authenticated. This can significantly impact your deliverability. Ensure that SPF, DKIM, and DMARC are properly configured and that your DNS records are correct.
• Blacklisting: If your sending IP address or domain is blacklisted, your emails are likely to be blocked by many mailbox providers. Check your IP address and domain against common blacklists using a blacklist lookup tool. If you are blacklisted, identify the reason for the listing and take steps to remove yourself from the blacklist.
• Broken Links: Broken links in your email can also trigger spam filters. Ensure that all links in your email are working correctly.
• Missing Alt Text: Missing alt text for images can also contribute to a higher spam score. Add descriptive alt text to all images in your email.

Taking Action Based on Results

Based on the interpretation of your spam filter results, you need to take specific actions to address the identified issues. Here are some common actions you can take:

• Content Optimization: Review your email content and remove or replace any words, phrases, or formatting issues that are triggering spam filters. Use a plain-text version of your email in addition to the HTML version.
• Authentication Fixes: Ensure that SPF, DKIM, and DMARC are properly configured and that your DNS records are correct. Test your authentication setup using an email authentication checker tool.
• Blacklist Removal: If your IP address or domain is blacklisted, identify the reason for the listing and take steps to remove yourself from the blacklist. This typically involves contacting the blacklist operator and providing evidence that you have addressed the issue that led to the listing.
• List Hygiene: Regularly clean your mailing list by removing invalid email addresses and unsubscribed users. Use a double opt-in process to ensure that users are genuinely interested in receiving your emails.
• Feedback Loop Management: Actively monitor and respond to feedback loops. Remove users who mark your emails as spam from your mailing list.
• Warm-Up Your IP Address: If you are using a new IP address for sending emails, gradually increase your sending volume over time to establish a positive sending reputation.

Example Scenario: Improving Deliverability After Spam Filter Analysis

Let’s say a seed list test reveals a high spam score and a DKIM failure. The report highlights the use of several spam trigger words in the subject line and body of the email. Additionally, the DKIM test indicates that the DKIM signature is invalid.

Action Plan:

• Content Optimization: Rewrite the subject line and body of the email to remove the spam trigger words. Use more natural and engaging language.
• DKIM Fix: Investigate the DKIM configuration. Verify that the DKIM key is properly generated and that the correct public key is added to the DNS settings. Use a DKIM checker tool to verify the DKIM signature.
• Resend Test: After implementing the content optimization and DKIM fix, resend the email to the seed list to verify that the spam score has decreased and that the DKIM authentication is passing.

Quote from Email Deliverability Expert:

“Email deliverability is an ongoing process, not a one-time fix. Regularly monitor your sending reputation, analyze your spam filter results, and adapt your strategies to stay ahead of evolving spam filters.” – John Smith, Email Deliverability Consultant

By consistently interpreting spam filter results and taking appropriate action, you can significantly improve your email deliverability and ensure that your messages reach their intended recipients. Remember to regularly monitor your sending reputation and adapt your strategies to stay ahead of evolving spam filters.