SPAM Filter 101: What It is and How to Avoid It

A spam filter is essential tool for managing personal and professional emails. They protect your inbox from unwanted messages.

According to Statista, in 2023 alone, 84.3% of all email traffic was spam . 

This means that without email spam filters, most of your messages would be junk.

For cold emailers, understanding how spam filters work is extremely crucial. These filters affect whether your emails reach potential clients. They also protect your own inbox from spam.

This blog will explain how email spam filters work. You’ll learn why they’re important and how to use them effectively. 

Whether you’re new to cold emailing or have been doing it for years, this information will help you send more emails to the prospects’ inboxes.

Let’s start with the basics.

What is a spam filter?

A spam filter, also known as “email spam filter”, is a software tool that screens incoming emails. It decides which messages are legitimate and which are spam. 

source

Spam filters protect your inbox from unwanted, harmful, or fraudulent emails.

They use complex algorithms to analyse each email based on factors such as – 

• Sender’s email address
• Email content
• Links in the email
• Email attachments
• Sending patterns etc.

Spam filters work in different places. They can be:

• Built into your email service (like Gmail or Outlook)
• Part of your company’s email server
• A separate program you install

Most modern email service providers have in-built spam filters. They work automatically, sorting emails before you even see them. 

When a filter thinks an email is spam, it usually does one of two things:

1. Moves the email to a spam or junk folder

                         OR

2. Blocks the email completely

But spam filters aren’t always perfect. Sometimes they might mark a good email as spam. 

This is why it’s important to check your spam folder regularly, especially if you’re expecting an important message.

What is a spam folder?

A spam folder is like a digital junk drawer for your email. It automatically collects unwanted or suspicious messages, keeping your inbox clean and focused on important emails. 

What are the different types of email spam filters?

Email spam filters come in various forms, each with its own unique approach of identifying and blocking spam. 

They can be classified based on two main categories: 

• Deployment and 
• Content analysis.

Email spam filters based on deployment

1. Gateway Filters

• These are the frontline defenders, operating at the network level.
• They screen emails before they even reach individual inboxes.
• Businesses and email service providers often use these filters.

Example: Barracuda Spam Firewall stands guard at the network entrance.

2. Client-side Filters

• These filters work directly on your device or email client.
• They’re like personal assistants, tailored to your preferences.
• Users can customize these filters to their liking.
• They’re the last line of defense, catching what slips past other filters.

Example: Microsoft Outlook’s built-in junk mail filter, Thunderbird’s spam filter etc.

3. Cloud-based Filters

• Operating on remote servers, these filters are always on duty.
• They offer scalability and real-time updates, adapting quickly to new threats.
• It’s like having a team of experts constantly monitoring your emails.
• They use the power of the cloud to protect your inbox.

Example: Google’s Gmail spam filter, Proofpoint Essentials etc.

4. Integrated Filters

• These are built right into email services or platforms.
• They work seamlessly with the email system, like a well-oiled machine.

Example: Yahoo Mail’s SpamGuard, Zoho Mail’s spam filter etc.

Email spam filter based on content analysis

1. Bayesian Filters

• These filters are the mathematicians of the spam-fighting world.
• They use statistical probability to identify spam.
• They learn from past experiences and improve over time.
• They get smarter with each email they process.

Example: SpamBayes 

2. Heuristic Filters

• These filters follow a set of rules to detect spam characteristics.
• They’re like experienced judges, weighing multiple factors.
• Admins can update them to counter new spam tactics.
• They adapt to new spam trends, staying one step ahead.

Example: SpamAssassin 

3. Blacklist/Whitelist Filters

• Blacklists are the “no-entry” lists for known spam sources.
• Whitelists are the VIP passes for approved senders.
• They often work together with other filter types.

4. Content-based Filters

• These filters are the detectives of the email world.
• They analyze email text, subject lines, and attachments.
• They look for spam-related keywords or suspicious patterns.
• Some even include image analysis to detect spam in graphics.
• They’re constantly on the lookout for telltale signs of spam.

5. Reputation-based Filters

• These filters check the sender’s credibility.
• They assess the IP address or domain reputation.
• They use databases of known spammers to make decisions.
• It’s like checking someone’s references before trusting them.

Example: Sender Policy Framework (SPF) is an example of this approach.

6. Machine Learning Filters

• These are the brainiacs of the spam filtering system.
• They use AI to adapt to new spam techniques.
• They’re always learning, improving accuracy based on data analysis.
• They’re the future of spam filtering, getting smarter every day.

 Example: Gmail’s AI-powered spam filter and Microsoft Defender for Office 365 use this technology.

Why are spam filters necessary for the email industry?

Spam email filters are crucial for the cold email industry, balancing sender needs with recipient protection.  They maintain email ecosystem health and ensure legitimate messages reach their targets.

Their goal is to create a more trustworthy email environment, encouraging cold emailers to focus on quality over quantity.

Deliverability is key for cold emailers. 

A study by Return Path found that 21% of legitimate emails never reach the inbox, often due to spam filters. This underscores the importance of understanding these filters.

For businesses, spam filters defend against phishing attacks. 

The FBI’s Internet Crime Report 2021 states that phishing scams cost U.S. businesses over $44.2 million. Effective filters help mitigate this risk.

Spam filters also help maintain sender reputation. 

EveryAction reports that 20.4% of commercial emails don’t reach the intended recipient.  A good sender reputation, cultivated by respecting filter rules, can significantly improve this statistic.

For recipients, spam filters increase productivity. 

Nucleus Research found that spam costs $712 per employee annually in lost productivity. Effective filtering reduces this cost.

In the B2B sector, spam filters ensure important communications aren’t lost. 

The Radicati Group predicts that by 2025, business emails sent and received per day will exceed 376.4 billion. Efficient filtering is crucial to manage this volume.

How does an email spam filter work? (Process explained)

Email spam filters use various techniques to identify and isolate unwanted messages.  

Understanding how these filters work is essential for anyone involved in cold email outreach. It helps in crafting emails that are more likely to reach the inbox, ensuring better email deliverability and engagement rates.

Let’s see how they work.

Step 1) Initial assessment

When an email arrives, the filter first checks the sender’s IP address and domain against known blacklists. If the sender is blacklisted, the email is immediately flagged as spam.

Step 2) Header analysis

The filter examines the email’s header information, looking for inconsistencies or suspicious elements. If an email fails multiple header checks, it is likely flagged as spam and moved to the junk folder.

This includes checking the ‘From’, ‘To’, and ‘Reply-To’ fields for mismatches or known spam patterns. 

For instance, a large number of recipients in the ‘To’ field might indicate a mass mailing and possibly a spam.  Also, spammers frequently use fake or misleading ‘From’ addresses to trick recipients.

Step 3) Content evaluation

The filter scans the email’s subject line and body text. It looks for:

• Spam trigger words and phrases
• Excessive use of capital letters or punctuation
• HTML code that might hide text
• Unusual character encoding

Step 4) Link and attachment scrutiny

Any links in the email are checked against databases of known malicious URLs. Attachments are scanned for potential malware and their file types are verified against allowed lists.

Step 5) Authentication checks

The filter verifies if the email passes authentication protocols like SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting, and Conformance).

Step 6) Machine learning analysis

Advanced filters use AI to learn from patterns in previously identified spam, continuously updating their criteria to catch new spam tactics.

Step 7) User behavior consideration

Some filters take into account how users interact with similar emails, using this data to refine their spam detection.

Step 8) Scoring system

Each element checked contributes to an overall spam score. If this score exceeds a certain threshold, the email is marked as spam.

Step 9) Final decision

Based on all these factors, the filter decides whether to deliver the email to the primary inbox, send it to the spam folder, or reject it entirely.

Here’s an Youtube video that describes the process – 

SPAM filters work differently for each email service providers.

While the basic principles are similar, each major email service provider has its unique approach to spam filtering, such as –

Gmail

Gmail uses a combination of rule-based filters and machine learning. It employs TensorFlow to detect subtle spam patterns. The spam detection is personalized based on individual user behavior. Gmail continuously updates its algorithms to combat emerging threats.

Outlook

Outlook utilizes Microsoft’s SmartScreen technology. It incorporates user feedback to improve filtering accuracy. The system employs heuristic filtering to detect suspicious patterns. Additionally, Outlook uses sender reputation scoring to evaluate email trustworthiness.

Yahoo Mail

Yahoo Mail relies heavily on user reports to identify new spam sources. It employs content-based filtering to analyze message text. Additionally, it uses DomainKeys and SPF to authenticate senders. Yahoo Mail also offers customizable filters for individual users.

These email providers constantly evolve their spam filtering techniques to stay ahead of spammers. 

They often use a mix of proprietary technologies and industry-standard protocols, making it crucial for email senders to adhere to best practices across all platforms.

Related: 8 Best FREE Email Service Providers | Create Free Emails

7 Factors that determine if your email lands in SPAM or not

Understanding what triggers spam filters is crucial for anyone sending emails, especially in the cold email industry. 

Here are some factors that influence whether your email reaches the inbox or to the spam folder.

Factor #1 ➡️ Sender reputation

Your email’s journey begins with your sender reputation. A poor reputation can signal spam filters that you are short of credibility as a sender.

Factors affecting this include –

• Inconsistent sending volume
• High email bounce rates. (Cheat Sheet added)
• Large number of spam complaints 
• High email unsubscription rates
• Domain reputation and more elements

Read more: Top 10 Strategies to Improve Email Sender Reputation

Factor #2 ➡️ Email authentication

Proper authentication is like showing your ID at a security checkpoint. Failing  checks like SPF, DKIM & DMARC can raise red flags to the spam filters.

Factor #3 ➡️ Content quality

Your email’s content plays a great role. Spam filters scrutinize – 

• Subject lines (avoid ALL CAPS and excessive punctuation!!!)
• Body text (stay clear of spammy words like “free”, “guarantee”, “no obligation”)
• Text-to-image ratio (too many images can be suspicious)
• Proper spelling and grammar (typos can trigger filters)
• Hosting for images and links
• Email formatting
• Text-to-code ratio (it should be less than 40%)

Read more: How to Write Cold Email Subject Line & Body Copy?

Factor #4 ➡️ Engagement metrics

How recipients interact with your emails matters. Low engagement can indicate your emails are not helpful to the recipients and spam filters can flag your emails as spam.

Read more: 15 Sales Email Metrics & KPIs to Track

Factor #5 ➡️ Sending practices

Poor email sending practices such as Inconsistent sending schedule, bulk email sending, invalid email addresses, sending emails with no option for opt-out etc. increases the chance of your emails getting flagged by spam filters as unsolicited.

Factor #6 ➡️ Link quality

Poor link practices in the email may push it to spam folder, including-

• The use of URL shortener for link attachments in the emails
• Use of links leading to spammy sites
• Overloading email body with too many links

Factor #7 ➡️ Attachments in the email

Attachments can be a minefield for spam which includes- 

• Sending uncommon file types (PDF, DOCX is the best)
• Sending large size files as attachments

Remember, spam filters are constantly evolving. What works today might not work tomorrow. 

4 Spam filter testing tools for checking deliverability issues

Let’s look at the some top-notch solutions that offer unique features to combat unwanted emails effectively. 

Each of these software options brings something special to the table, catering to different needs and preferences.

1. SpamTitan

SpamTitan stands out as a comprehensive email security solution, offering both cloud and gateway options. 

With its impressive 99.99% spam detection rate and low 0.03% false positive rate, it’s a reliable choice for businesses of all sizes. 

SpamTitan’s dual-layer virus protection and advanced threat detection capabilities make it a formidable defense against phishing and ransomware. \

Its user-friendly interface, detailed reporting, and features like geoblocking and outbound email scanning provide a well-rounded approach to email security.

2. Barracuda Email Security Gateway

Barracuda’s Email Security Gateway is an all-in-one appliance that tackles every aspect of email protection. 

It excels in inbound and outbound filtering, data loss prevention, and encryption. What sets it apart is its cloud-based email continuity service, ensuring uninterrupted access to your emails even during server downtime. 

With its intuitive management interface and strong focus on regulatory compliance, Barracuda is particularly valuable for organizations in highly regulated industries.

3. Mailwasher

Mailwasher takes a unique pre-emptive approach to spam filtering. By allowing users to preview and delete emails directly from the server, it not only saves bandwidth but also adds an extra layer of security. 

Its learning capabilities mean it gets better at filtering over time, adapting to your preferences. 

Mailwasher’s standout feature is its highly customizable filtering system, giving users granular control over their email. This makes it an excellent choice for individual users and small businesses looking for a hands-on approach to spam management.

4. Spam Sieve

Spam Sieve is a powerhouse spam filter tailored specifically for macOS users. Its seamless integration with popular email clients like Apple Mail and Outlook makes it a go-to choice for Mac enthusiasts. 

Spam Sieve’s combination of Bayesian filtering and thousands of built-in rules results in exceptionally accurate spam detection. 

The software’s high degree of customization, including the ability to create personal rules and train the filter, puts users in control of their email experience. 

Features like automatic whitelist management and color-coded spam flags make Spam Sieve both powerful and user-friendly.

We have listed more tools in our article “10 Tools To Check Sender Score (Free + Paid)”. Make sure you check that out!

How to avoid SPAM filters for cold email campaigns

Spam filters don’t like it when you do a cold email. 

Here’s an in-depth guide to help your emails reach the inbox consistently. These practical strategies will boost your deliverability and campaign success.

I have used one of the best cold email software SmartReach for this example. 

Tip #1. Use a reliable email service provider

Choose a reputable email service provider that has good deliverability records. You may choose ESPs such as Gmail, Outlook, Yahoo mail etc. 

💡 Tip: If you running cold email outbound, Outlook is recommended for a better deliverability.

Tip #2. Choose a good email automation software

Use a good email automation software for running the cold email outreach on automation. Make sure it includes – 

• Built-in deliverability features
• Good relationships with major email inbox providers
• Compliance with email regulations

As a cold email automation software SmartReach provides:

• Inbox rotation for safely scaling up email sending volume ✅
• Automatic handling of bounces and other deliverability issues ✅
• Inbox placement report for monitoring the real time email deliverability ✅

Action step ➡️ Set up your SmartReach account and familiarize yourself with these deliverability features.

#2. Always warm up your email domain

Gradually increase your sending volume over time. This process is crucial for building a good sender reputation.

Steps for effective domain warming:

1. Start with small batches (10-20 emails per day)
2. Increase volume by 10-20% every week
3. Aim for positive engagement (opens, replies) during this period
4. Continue until you reach your desired daily volume

SmartReach offers an automated warm-up feature that

• Gradually increases your email sending volume ✅
• Simulates positive interactions to boost your reputation ✅
• Adapts the warm-up process based on recipient reactions ✅

Action step ➡️ Set-up SmartReach’s warm-up tool Warm-up hero and monitor your progress over 4-6 weeks.

Tip #3. Personalize your emails as much as you can

Spam filters look for signs of mass-sent emails. Email personalization helps avoid this and improves engagement.

Effective personalization techniques:

• Use the recipient’s name in the subject line and greeting
• Reference the recipient’s company or industry
• Mention a recent achievement or news about their business
• Customize the email body based on the recipient’s role or interests

SmartReach’s personalization suite of features allow you to:

• Create dynamic email content that change based on recipient data you put ✅
• A/B test different personalization strategies ✅
• Create email sequences based on contexts with the help of AI ✅

Action step ➡️ Create AI- generated email sequences with at least 3-4 personalized elements using SmartReach’s personalization features.

Tip #4. Craft a clear subject line

Your subject line is crucial for both avoiding spam filters and encouraging opens.

Subject line best practices:

• Keep it under 40-50 characters
• Avoid spam trigger words (e.g., “free”, “guarantee”, “no obligation”)
• Use action-oriented language
• Create a sense of relevance or urgency
• Don’t use all caps or excessive punctuation (e.g., %,$$,!!)

For the examples of subject lines, check out “165 B2B Cold Email Subject Lines For Sales Outreach”

Action step ➡️ Use SmartReach to AI generate subject lines lines using prompts. And A/B test email subject lines whenever possible for new cold email outreach campaigns.

Tip #5. Balance text and images

Too many images can trigger spam filters and affect deliverability.

Guidelines for image use in emails:

• Aim for a text-to-image ratio of at least 80:20
• Always include alt text for images
• Avoid sending image-only emails
• Host images on a reputable server
• Compress images to reduce load time and attachment size limit
• Do not include image until it is absolutely necessary. 

Action step ➡️ Review your email templates before hitting the “send” button and adjust the text-to-image ratio as needed.

Tip #6. Include an unsubscribe link

Providing an easy way to opt out is not just good practice, it’s often required by law. It helps you keep the spam complaints low.

Best practices for unsubscribe links:

• Place the link in a prominent, easy-to-find location
• Make the unsubscribe process one-click when possible
• Honor unsubscribe requests promptly (within 3 business days)
• Consider offering subscription management options 

SmartReach automatically:

• Includes an unsubscribe link in your emails ✅
• Processes unsubscribe requests immediately with a DNS status ✅
• Auto-Update your email list to exclude unsubscribed contacts ✅
• Customizeable unsubscribe button/link ✅

Action step ➡️ Test your unsubscribe process with a dummy email to ensure it works smoothly.

Tip #7. Clean your email list regularly

A clean list improves delivery rates and sender reputation.

Email List cleaning process:

1. Remove hard bounces immediately
2. Remove soft bounces after multiple failed attempts
3. Remove or re-engage inactive subscribers (no opens/clicks for 6+ months)
4. Verify email addresses before sending

SmartReach offers:

• Automatic email bounce handling option for both hard and soft bounces ✅
• Free in-built email validation tool to identify and remove invalid emails ✅
• Re-engagement campaign templates for inactive subscribers ✅

Action step ➡️ Monitor the email list while uploading new set of prospects to the campaign.

Tip #8. Send few email attachments

Attachments can trigger spam, especially in cold emails.

Use these alternatives to sending email attachments 👇🏻

• Use links to cloud-hosted documents (e.g., Google Docs, Dropbox)
• Include key information directly in the email body
• Offer to send additional materials upon request

If you must use attachments –

• Stick to common file types (PDF, DOCX) ✅
• Keep file sizes small (under 5MB if possible) ✅
• Clearly describe the attachment in your email ✅

Action step ➡️ Review your email templates and replace attachments with links where possible.

Tip#9. Monitor your email metrics

Keep a close eye on your email performance metrics to identify and address issues quickly. 

Key metrics to track for cold emails:

• Open rates 
• Click-through rates (avoid link tracking if possible)
• Bounce rates 
• Spam complaint rates 
• Reply rates (Cheat Sheet linked)

SmartReach provides a lot of options for tracking your email campaigns including –

• Detailed analytics dashboard for email campaign performance ✅
• Real-time performance tracking for each email campaign ✅
• Comparative analysis across campaigns ✅
• Automated alerts for high spam rates or any other deliverability issues ✅

Action step ➡️ Set up weekly performance reviews using SmartReach’s analytics. Identify trends and adjust your strategy accordingly.

By implementing these detailed strategies and leveraging SmartReach’s features, you can significantly improve your email deliverability and avoid 

Start your 1st campaign for FREE using SmartReach today. (No credit card required)

If you have any questions, doubts etc. regarding the tool, its use case you can book a FREE consultation call.

Email Spam Filter: FAQs

Q. What is a spam filter?

A spam filter is a software application that automatically detects and blocks unsolicited, unwanted, or harmful emails (spam). It protects users from phishing scams, malware, and simply overwhelming their inbox with junk mail.

Q. How to avoid spam filter?

To avoid spam filters, focus on these key points:

• Authenticate your domain
• Maintain a clean email list
• Avoid spam trigger words
• Use a clear and engaging subject line
• Honor opt-out requests promptly
• Optimize image-to-text ratio
• Warm up your IP address
• Use a reputable email service provider (ESP)
• Monitor your email deliverability
• Consider using a spam testing tool
• Personalize your emails
• Avoid excessive links

Q. How does gmail spam filter work?

• Machine learning: Identifies patterns in spam emails.
• Sender reputation: Evaluates sender’s history and trustworthiness.
• Content analysis: Checks for spammy words, links, and images.
• User feedback: Learns from user actions (marking as spam/not spam).
• IP address analysis: Detects suspicious IP addresses.
• Domain reputation: Evaluates domain’s history for spam.
• Authentication checks: Verifies sender identity (SPF, DKIM, DMARC).
• Bayesian filtering: Calculates probability of email being spam.

Q. How does gmail spam filter work?

Gmail’s spam filter uses machine learning, sender reputation, content analysis, user feedback, and IP checks to identify and block unwanted emails.

Q. How to check spam filter in gmail?

To check your Gmail spam folder:

1. Open Gmail.
2. Look for “More” in the left sidebar.
3. Click “Spam” to view filtered emails.
4. If it’s not there, go to Settings -> Labels -> Show next to Spam.

Q. How to test spam filter?

To test your spam filter:

• Use a test email account: Send emails with spam-like content to this account.
• Use spam testing tools: Online services like Mail-Tester can analyze your email.
• Monitor your spam folder: Regularly check for legitimate emails ending up there.
• Utilize GTUBE: This standardized spam signature forces filters to recognize it as spam.
• Analyze filter logs: If available, review logs for insights into filter behavior

Q. How do I clear my spam filter?

Clearing your spam filter isn’t straightforward. You can’t completely reset it. However, you can:

• Mark emails as “Not Spam”: This teaches Gmail what isn’t spam.
• Create filters: Move specific emails away from spam.
• Delete spam regularly: This prevents clutter.
• Avoid clicking links or downloading attachments: This reduces spam triggers.

Q. What does a spam filter do?

A spam filter protects your inbox by identifying and blocking unsolicited, unwanted, or harmful emails. It sorts through incoming messages, categorizing them as legitimate or spam, and preventing spam from reaching your main inbox.

Q. How to disable spam filter in gmail?

You cannot completely disable Gmail’s spam filter. It’s a core part of Gmail’s security system. However, you can:

• Create filters: To move specific emails away from spam.
• Mark emails as “Not Spam”: To teach Gmail what’s legitimate.

Q. How do I empty my spam filter list?

To empty your Gmail spam folder:

1. Open your Gmail account.
2. Click on the “Spam” label in the left-hand menu.
3. Select all the emails in the spam folder.
4. Click the “Delete” button.

Q. How to clear spam filter list on android?

One can’t fully clear Android spam list. An user can delete individual spam, mark important as “not spam”. Also, in this case, some third-party apps might help.