How to avoid spam filters: 12 techniques that actually work [2026]

One in six permission-based marketing emails fail to reach the inbox of intended audiences, as reported by Validity’s 2025 Email Deliverability Benchmark Report. For many businesses, this means losing a significant percentage of potential business prospects. 

While spam filters play a major role in automatically managing unwanted emails, their increasing strictness can cause valid emails to land in the spam folder, never reaching intended recipients. 

There are specific actions you can take to ensure your emails reach recipients. In this guide, we’ll go over 12 different techniques to avoid spam filters, all centered around 4 key stages — setup, creation, testing, and monitoring. 

What is a spam filter?

An email spam filter is a security tool that screens incoming messages to detect and quarantine harmful or unwanted emails. Email providers like Gmail, Yahoo Mail, and Outlook use these filters to protect inboxes from malware, phishing attempts, and unsolicited bulk emails.

Currently, there are four main types of spam filters that directly affect your campaign performance and ROI. These filters analyze sender reputation, email content, and engagement to determine if your emails are legitimate and fit to reach subscribers’ inboxes.

For your marketing campaigns to succeed, your emails must clear these spam filters to reach the inbox where recipients can engage with your content.

Messages caught by spam filters land in junk folders where subscribers may never see them, wasting your marketing efforts and killing potential sales. You should treat every unsuccessful email as a missed opportunity to connect with customers and drive revenue.

Spam filter vs. spam folder: what’s the difference?

These terms are often used interchangeably, but in reality, they describe different things. For starters, spam filters are decision-based systems that set pre-defined rules and algorithms for evaluating individual emails. 

A spam folder is simply a dedicated location for all spam-flagged emails to land, depending on how mailbox providers set up spam filters. While spam filters can be pretty strict, most providers offer a spam folder as a way to double-check content and allow recipients to decide whether they want to move certain emails from spam folders to their main inboxes. 

How do spam filters work? The 4 layers of modern filtering

To put it simply, spam filters run your email through several checks to determine whether the email should reach recipients or be sent to spam folders. But it’s important to note here that these filters don’t make a single definitive decision, like blocking your emails.

The full verification process involves going through 4 stages: authentication, sender reputation, email content, and engagement rate.

1. Authentication filters (SPF, DKIM, DMARC)

Authentication filters play a key role in deciding where emails should go. Mailbox providers check the identity of all senders to make sure no one is using someone else’s identity or company name. In this sense, this stage can also be considered a security check. By default, most authentication filters verify three Domain Name System (DNS)-based records:

• Sender Policy Framework (SPF): confirms whether the sending server is authorized to send emails for your domain
• DomainKeys Identified Mail (DKIM): adds a cryptographic element, or signature, which serves as proof that your email wasn’t altered during the sending process
• Domain-based Authentication, Reporting, and Conformance (DMARC): gives clear instructions to mailbox providers whether to quarantine or reject your email if SPF or DKIM checks fail

To make sure your emails pass all three checkpoints of the authentication process, you need valid SPF domains, a verified DKIM signature, and at least a p=none DMARC policy. 

2. Reputation filters (sender + IP)

These filters check your IP address and domain to find out if you have a high or low complaint rate. Reputation takes a considerable time to build, so reputation filters look at whether you’ve previously sent emails to invalid addresses or if you’ve shared an IP address with spammers.

In short, to pass the reputation test, you have to use official Internet Service Provider (ISP)-issued IP addresses, preferably with a and consistent sending volume. 

3. Content filters (Bayesian and keyword-based)

Content analysis is another important step. The key purpose here is to analyze your email text, images, and structure. Bayesian filters help decide whether your email message is spam based on statistical probability and established patterns. 

Keyword-based filters catch and flag specific phrases like “free money”, “click here”, “act now”, and others that often appear in spammy emails. Moreover, your email HTML structure is also important — if it’s broken, missing plain-text alternatives, or uses excessive red text, your email will likely be sent to spam.

Use structured HTML, balance text and visuals, include a plain-text version, and write clear email copy without relying on common trigger words. 

4. Engagement filters (the modern dominant signal)

Email deliverability trends for 2026 show that engagement metrics now play an important role in influencing whether emails land in spam folders or inboxes. 

Major mailbox providers, Gmail in particular, track how recipients interact with emails from your domain by looking at Click-Through Rate (CTR), Click-to-Open Rate (CTOR), read rate, and whether recipients open your emails or mark them as spam. 

To go through the 4 stages and pass engagement filters, you should have consistent opens, occasional replies, low spam complaint rates, and active recipients. 

Good signals vs. bad signals: what filters reward and punish

What triggers spam filters? The 8 most common causes

Many factors can trigger spam filters, but the eight most common are: missing authentication records, a damaged sender reputation, spam trigger words in subject lines or body copy, an imbalanced text-to-image ratio, a purchased or stale list, high bounce rates, low recipient engagement, and sending from a free email domain. 

Here’s what each one looks like in practice, and which precise techniques presented later in the article you can use, listed in this article, to fix or remove these causes:

1. Missing or misconfigured authentication: If your emails fail SPF, DKIM, or DMARC checks, it means your domain can’t prove ownership of the emails you send. Fix this with technique 1.
2. A damaged sender reputation: Having any past spam complaints, high bounce rates, or appearing on email blocklists can lead to your IP being flagged. To avoid this, use technique 12. 
3. Spam trigger words in subject lines: Words and phrases like “free gift”, “you’ve been selected”, “limited time offer” quickly trigger spam filters, but preventing this issue isn’t just about using the right words — read more about how to treat this issue with techniques 6 and 7. 
4. Image-heavy or image-only emails: Emails that contain over 90% of visuals and minimal text usually signal content filters that there’s not much to evaluate in your email, so the email is treated like spam. For this particular issue, use technique 8. 
5. A purchased, rented, or stale list: Emails without a supporting opt-in feature can include spam traps, such as blocklisted addresses, so try techniques 5 and 11 to fix this problem.
6. High hard bounce rates: Many startups send emails to large numbers of recipients, regardless of whether the addresses are valid, resulting in high bounce and unopened rates. Scroll to technique 11 to improve engagement filters. 
7. Low recipient engagement: If recipients consistently ignore your emails, the engagement filter will most likely classify them as unwanted. Review technique 11.
8. A free email domain in the “From” field: Despite the provider size, emails with @gmail.com, @yahoo.com, or @hotmail.com will often bypass domain-level reputation entirely and almost immediately trigger suspicion. Consider using technique 3 for this situation. 

How to avoid spam filters: 12 techniques

Avoiding and preventing spam filters boils down to four key stages: proper email infrastructure setup, clean content, regular email testing, and email performance tracking over time.

Set up your sending infrastructure

Using the right infrastructure is the foundation of any successful email campaign, and it’s the most straightforward way to prevent triggering spam filters. 

Technique 1: Authenticate your sending domain (SPF, DKIM, DMARC)

First things first, start by adding an SPF record to your DNS to authorize your sending service’s IP. Once you have SPF in place, enable DKIM signing to make sure all your emails have a verifiable signature. After that, set a DMARC policy with p=none for monitoring. When you’re ready to enforce stricter rules, move to p=quarantine, and eventually to p=reject for full email security enforcement.

If you’re using Omnisend, we have built-in features for setting up SPF, DKIM, and DMARC directly from inside our platform. You can read more about how to do this in our guided setup article. 

Technique 2: Meet the 2024 Google, Yahoo, and Microsoft sender requirements

Yahoo and Google introduced mandatory sender requirements in 2024. These requirements essentially state that anyone sending more than 5K messages per day needs to have:

• valid SPF, DKIM, and DMARC records, 
• a fully functioning one-click unsubscribe header element (RFC 8058),
• and a spam complaint rate below 0.10%. 

Following this, Microsoft also introduced similar requirements for Outlook Mail in 2025. Overall, these requirements are necessary if you want to make sure your emails aren’t filtered and rejected. 

Technique 3: Send from a business domain, not a free address

Try to use your business or brand domain ([email protected]) and not a free “From” email address ([email protected]). This is because free-provider addresses don’t have SPF or DKIM authority to use for your business.

On top of spam filters immediately being suspicious of free emails, sending emails from your domain helps build sender reputation and configure DMARC. This way, mailbox providers can evaluate you as a consistent identity.

Technique 4: Warm up new IPs and domains gradually

Don’t use new IP addresses and domains to send large email volumes. While they may not be tied to anything or appear on blocklists, in the eyes of spam filters, new IPs and domains often look suspicious and are treated as higher-risk senders. 

To avoid this, warm up your IP and domain by sending small daily email batches to your most engaged readers only. Double the batch size after a few days, then repeat the same after that for four to eight weeks. This gives mailbox providers time to evaluate you while you build engagement metrics. 

Create emails that don’t trigger filters

After setting up the basis for a strong infrastructure for your emails, the next step is to plan out and polish email texts to make sure they pass content and engagement filters. 

Technique 5: Use double opt-in and never buy lists

While not a must-have requirement globally, double opt-in can be a massive upgrade to your email systems. The double opt-in feature adds another confirmation layer by asking your recipients and subscribers to confirm their email addresses. 

This essentially takes care of a major problem from the get-go — verifying email addresses to avoid sending emails to invalid or inactive recipients. Moreover, double opt-in also helps reduce spam complaints, hard bounces, and spam trap hits. 

Technique 6: Write subject lines that pass

Write genuine subject lines that speak to your readers. Avoid using too many words, excessive amounts of punctuation marks like ??!!, or monetary symbols like $,€, or £. A good rule of thumb is to think of a subject line as something you’d personally be interested in for your niche and keep it under 60 characters.

To kick it up a notch, use personalization — Forbes reports that around 80% of people are more likely to interact with emails if they use real names and other personalized elements. Just make sure to always provide accurate information that doesn’t mislead readers, because that can and will backfire in the form of immediate closes. 

Technique 7: Avoid spam trigger words in body copy

Spam filters are highly trained to spot trigger words, which will immediately result in emails being sent to spam folders. Common phrases and words that increase spam scores include “act now”, “click here”, “free”, “guaranteed”, “no risk”, “winner”, “special promotion”, and similar variations. 

Creating urgency and scarcity are legitimate strategies, but you need to be careful how you structure your emails. You can already run a quick check by looking through previously sent emails — if any of them have trigger words, chances are those emails went to spam. 

Technique 8: Balance text and images (and skip attachments)

Using images and supporting visuals can seriously help with email deliverability, but overcrowding your emails with them can result in triggering spam filters. The industry standard for a well-balanced image text-to-image ratio is to use 60% text and 40% image. 

It doesn’t matter if you use high-quality and visually appealing images — if they’re not balanced with appropriate text, your emails will go to spam folders. Moreover, attachments are a surefire way to trigger spam, so if you need to share anything, link to a hosted version.

Technique 9: Keep HTML clean and provide a plain-text alternative

Export a clean HTML version from your email builder without leftover comments, inline CSS from a web context, or buried tables that can’t be rendered correctly. Every new campaign you launch should include a plain-text Multipurpose Internet Mail Extensions (MIME) part with the HTML version. 

Having a plain-text version will make your emails readable in environments that block HTML and show spam filters that you’re a legitimate sender. Most email platforms generate this version automatically, but check if your provider does that. 

Test before you send

While obvious, tests are crucial for not just quality checks — testing can make all the difference in preventing your emails from going to spam folders. 

Technique 10: Run a pre-send spam score check

One of the most popular ways is to use mail-tester.com to send test messages to the provided address to see your email score out of 10. Besides scoring your emails, the tool will also provide a pretty detailed breakdown showing what elements of your emails can pass spam filters, and which ones could flag them. 

Besides that, mail-tester.com also checks authentication, content, blacklist status, and other factors in under two minutes. If you want more information, try using services like GlockApps, which offer inbox placement testing across all major providers, including Gmail, Outlook, and Yahoo.

Monitor and maintain deliverability

Just like testing can help catch problems early on, monitoring your email performance consistently is essential for successful email campaigns. 

Technique 11: Clean inactive subscribers regularly

It’s always better to have a list of verified addresses rather than a massive list of unverified and disengaged recipients. More than that, inactive subscribers can really damage your reputation. 

To regularly check your recipients, try setting up a re-engagement window. If your subscribers don’t open your emails within 90 to 180 days, send the re-engagement email, and if you still get no response, remove those recipients. This way, you’ll protect your engagement rate, reduce complaint rates, and keep bounce rates low.

Technique 12: Monitor sender reputation in Postmaster Tools

Use reliable and free tools like Google Postmaster Tools to check your domain reputation, spam complaint rate, authentication pass rates, and delivery errors. All of this information is updated daily, so you can set up alerts to quickly view sudden drops or spikes. 

If you’re using Outlook, you can use Microsoft’s Smart Network Data Services (SNDS), which provides comparable data for your sending IP address. 

How to avoid the Gmail spam filter (and Outlook, Yahoo)

Every major mailbox provider evaluates the four main spam filters differently. For example, Gmail focuses quite heavily on engagement rates, Outlook emphasizes authentication and sender policies, while Yahoo weighs in on domain reputation history. 

Understanding these differences will make it easier for you to focus on the signals your provider prioritizes. 

Gmail-specific signals

Gmail is built around engagement, so to ensure your emails reach recipients’ inboxes, you should have a strong domain with a proven engagement history showing that subscribers open, click, and reply to your emails. Here’s what to focus on for Gmail: 

• Authenticate DMARC with p=quarantine or p=reject
• Keep your email complaint rate below 0.10%
• Enable one-click unsubscribe feature
• Send emails to engaged recipients only 

Outlook and Yahoo specifics

Among all major mailbox providers, Outlook is the one that puts the biggest emphasis on authentication. For that, Microsoft’s 2025 policy requires senders to have DKIM and SPF to pass spam filters. Moreover, Outlook also monitors your SNDS IP status. 

Yahoo focuses on domain reputation. You need to have a consistent list of clean emails sent over a period of at least six months to gain good inbox placement. Conveniently, Yahoo provides a feedback loop that can show specific spam complaints you can use as feedback to fix your future emails. 

How to test if your emails are landing in spam

There are three main tools you can start using now to check your emails before sending, monitor performance, and run inbox placement audits. 

1. mail-tester.com: a free and quick testing tool for pre-send verification. Allows you to send a test email to a unique testing address and see what your email sender reputation is. 

2. Google Postmaster Tool: another free tool that spans both pre-send and post-send, showing domain reputation, spam complaint rate, authentication pass rate, and delivery errors. 

3. GlockApps: a paid tool used for pre-send inbox placement testing. GlockApps sends your emails to a seed list on Gmail, Outlook, Yahoo, and other providers to show whether the emails land in the intended recipient’s inboxes. 

Frequently asked questions

What words trigger spam filters?

The most common triggers are different variations of “free”, “guaranteed”, “act now”, “click here”, “you’ve been selected”, “no risk”, “winner”, “limited time”, and “special promotion”. Other than that, using all-caps and a lot of punctuation signs can send emails to spam folders. 

Why are my emails going to Gmail spam specifically?

All major mailbox providers include all spam filters, but focus on specific ones. Gmail emphasizes engagement rates, so to make sure you pass spam filters with Gmail, you should have a low complaint rate that doesn’t exceed 0.10%. 

To avoid landing in Gmail’s spam folder, use tools like Google Postmaster Tools to get more data on your engagement issues and tighten your recipient lists to active subscribers.

How do I check if my emails are going to spam without asking recipients?

To check if your emails are going to spam without reaching out to recipients, use inbox placement testing tools like GlockApps. These tools will send your emails to a seed list of real mailboxes and give you more info on precisely where those emails go — to the inbox or spam folder. 

Do images cause emails to go to spam?

No, images don’t trigger spam alerts, but using too many of them certainly will. Any email that’s made up entirely of images without any text will signal to content filters that there’s nothing to analyze. In turn, content filters will treat this as an ambiguous case, triggering spam filters. 

To avoid this, use the 60/40 rule to balance images and texts, always include alt attributes, and don’t send emails with a single image only. 

Will my transactional emails go to spam, too?

Transactional emails usually contain information on order confirmations, shipping notifications, password resets, and other order or account-related information. In other words, it’s vital information that recipients want to open and read. 

Even so, if these emails don’t have proper authentication or are sent from a shared IP with a poor reputation, transactional emails can also go to spam. 

Do free email domains get filtered more often?

That’s right. Sending marketing or business emails from addresses @gmail.com, @yahoo.com, or @outlook.com makes it nearly impossible to configure domain-level SPF, DKIM, and DMARC because those records belong to Google, Yahoo, and Microsoft, so use business domains you can control. 

How long does it take to fix a damaged sender reputation?

Fixing a damaged sender reputation depends on how damaged it is. 

• If you’re experiencing a minor sender reputation drop from singular complaints, your sender reputation will likely stabilize within a few weeks
• If you’ve been blocklisted, you’ll need to go through a formal process of sending a request to be delisted
• If you have a severely damaged sender reputation, it could take anywhere from three to six months to fix

Are unsubscribes bad for deliverability?

No, they can even help keep your lists clean with active subscribers only. When a recipient unsubscribes, it doesn’t affect your email deliverability. But if they click on “report spam”, this could become a problem.