You dependent over every email campaign — carefully crafting the perfect copy, agonizing over subject lines, and tweaking design elements for maximum impact. But then, despite your blood, sweat, and tears, your beautiful creation never even sees the inside of your subscribers’ inboxes. It’s banished to the spam folder, never to be opened or clicked.
So what gives? Why are spam filters so trigger-happy with your campaigns, even when you’re not selling shady pills or running Nigerian prince scams?
What Are Spam Filters and How Do They Work?
It all comes down to how spam filters work behind the scenes. Every major email provider (Gmail, Outlook, Yahoo, etc.) uses sophisticated algorithms to analyze incoming mail and weed out suspected spam.
These digital detectives look at hundreds of factors related to your sender reputation, email content, and subscriber engagement to decide if you’re worthy of the inbox. Stuff like:
- How many emails are marked as spam by recipients
- Whether your sending IP address is associated with known spammers
- If your email copy contains spammy words or phrases
- How many invalid email addresses you’re sending to
- The ratio of image-to-text in your email
- How recently and consistently subscribers have been engaging with your emails
Spam filters aren’t just looking at individual red flags — they’re looking at the big picture of your email program. One “oops” might not mean much, but a pattern of suspicious behavior can relegate you to the junk folder faster than you can say “unsubscribe.”
And here’s the kicker — every email provider has their own unique spam filtering recipe. So the tactics that keep you on Gmail’s nice list might not pass muster with Outlook. Crazy, right?
If you want to keep your emails out of spam purgatory, you need a holistic approach to email deliverability — one that covers all the potential risk factors and red flags.
Let’s dive into the 18 most common reasons emails get flagged as spam and what you can do to avoid the wrath of spam filters. By the time we’re through, you’ll be confidently sending emails that land in the inbox consistently.
1. Your Sender Identity is Questionable
So, imagine you send an email without proper authentication protocols in place. The spam filter takes one look at your email and laughs. Without authentication, your emails look suspicious.
What is email authentication? It’s a way to prove your identity to email providers and assure them you’re not someone spoofing another person’s address. The key protocols are:
- SPF (Sender Policy Framework): Specifies which IP addresses are allowed to send emails from your domain.
- DKIM (DomainKeys Identified Mail): Adds a digital signature to your emails to verify the sender.
- DMARC (Domain-based Message Authentication, Reporting & Conformance): Tells email providers what to do with emails that fail SPF or DKIM checks.
If you haven’t set these up, your emails have a much higher chance of getting flagged as spam. But don’t worry, it’s not as complicated as it sounds.
How to Set Up Email Authentication:
- Work with your IT team or email service provider to configure SPF, DKIM, and DMARC records for your sending domain.
- Use tools like Google Postmaster or 250ok to verify your records are set up correctly and troubleshoot any issues.
- Consider implementing BIMI (Brand Indicators for Message Identification) to display your logo next to authenticated emails. It’s an extra trust signal for recipients.
Some popular email marketing software options include Mailchimp, SendGrid, and ActiveCampaign, all of which offer features that help avoid spam filters while optimizing email deliverability.
2. Your Sender Reputation Needs Improvement
Your sender reputation is like your credit score for email. It takes time and consistent positive behavior to build, but a few slip-ups can damage it fast.
Email providers judge your reputation based on factors like:
- Spam complaints: If too many people mark your emails as spam, it’s a major red flag.
- Hard bounces: Sending to too many invalid or closed email addresses damages your reputation.
- Engagement metrics: Low open rates and high unsubscribe rates tell providers your emails aren’t wanted.
If your sender reputation falls into the gutter, even your best emails will have a hard time reaching the inbox. But with some consistent effort, you can improve your reputation and reap the deliverability rewards.
How to Improve Your Sender Reputation:
- Use double opt-in to ensure subscribers actively choose to be on your list.
- Purge inactive subscribers and invalid email addresses from your list regularly.
- Send relevant, engaging content that inspires opens and clicks (more on that later).
- If you’re on a shared IP address, consider moving to a dedicated IP so your reputation isn’t affected by other senders’ actions.
- Monitor blacklists and spam traps, and take immediate action if you find yourself on one. Tools like MXToolbox are great for this.
There’s no quick fix for a tarnished sender reputation, but every positive sending practice helps nudge the needle in the right direction. Make reputation management an ongoing priority.
3. Your Subject Lines Look Like Spam
You know those subject lines that practically scream “I’m spam!” — the ones that make outrageous claims or use shady tactics to trick you into opening? Yeah, spam filters can spot those a mile away.
If your subject lines consistently set off spam filter alarms, your deliverability is going to suffer. The spam filters will take one look and send you packing.
But that doesn’t mean your subject lines have to be boring! You can still pique curiosity and inspire opens, but you need to do it in a way that feels authentic and delivers on its promises.
How to Craft Non-Spammy Subject Lines:
- Ditch the “RE:” and “FWD:” tricks. They might boost opens in the short term, but they’ll destroy your reputation in the long run.
- Avoid using all caps, excessive punctuation (!!!), or overtly sales-y phrases (“Act now!”, “Free!!”).
- Personalize your subject lines with the recipient’s name or location to show you’re not just blasting the same message to everyone.
- A/B test different subject line approaches and stick with the ones that get the best engagement.
- Keep it concise — aim for 50 characters or less so it doesn’t get cut off on mobile.
Your subject line makes a promise to the reader about what’s inside the email. Make sure you deliver on that promise and don’t resort to bait-and-switch tactics. Consistency builds trust, and trust is key to the inbox.
4. You’re Using Spam Trigger Words
“FREE!” “Guarantee!” “Act now!” — these words might seem like persuasion gold, but to spam filters, they’re giant red flags.
Spammers have ruined them for the rest of us by using them excessively in their shady emails. Now, even legitimate senders need to use them sparingly to avoid getting penalized.
But that doesn’t mean you have to strip your emails of all personality. You can still write compelling copy; you just need to get a little creative with your word choice.
How to Avoid Spam Trigger Words:
- Swap hype-y words like “amazing,” “free,” or “act now” with more understated alternatives (“helpful,” “complimentary,” “limited time”).
- Focus on benefits, not bragging: Instead of “we’re the best,” try “see how we’ve helped customers like you.”
- Tell a story: Instead of “90% off sale,” set the scene: “Imagine stretching out on the beach, enjoying the vacation you scored for a fraction of the usual cost…”
- Avoid gratuitous dollar signs and “click here!” call-outs. Stick to one or two at most.
Better yet, have a trusted friend or coworker read over your copy and point out any phrases that feel overused or inauthentic. A little outside perspective can help you steer clear of accidental spam triggers.
5. Your Design Makes Filters Cringe
Your design choices don’t just affect how your emails look — they also play a big role in whether or not you land in the spam folder. Overly busy designs littered with images, messy code, and not enough context-providing text are instant turnoffs for spam filters.
How to Improve Your Email Design:
- Aim for a balanced text-to-image ratio (60:40 is a good rule of thumb). Spam filters can’t “read” images, so they rely on accompanying text to determine relevance.
- Make sure you have alt text for every image, just in case the recipient has images disabled. This also gives spam filters more context.
- Optimize your image sizes to keep your email from getting too “heavy”. Sizle and TinyPNG are great tools for this.
- Use a reputable email template designer or have a developer check your code for any red flags (like funky tags or excessive inline CSS).
- Test your email across multiple devices and email clients to make sure everything looks and functions as intended. Litmus and Email on Acid are go-to tools for this.
A clean, mobile-friendly design goes a long way toward building trust with both spam filters and recipients. If your email looks like it was cobbled together in 1999, you’ll struggle to reach the inbox. Keep it simple, streamlined, and scannable.
6. Sending Emails Inconsistently
Imagine you had a friend who would go radio silent for months, then randomly show up at your door in the middle of the night, talking a mile a minute. You’d probably think they were a little unhinged, right?
Well, that’s how spam filters see senders who email sporadically with no consistent schedule. Erratic sending patterns — like going from 0 to 100,000 emails overnight — are a huge red flag for filters.
How to Achieve Consistent Sending:
- Create a sending schedule and stick to it, whether that’s every week, every other week, or once a month. Filters like predictability.
- If you’re ramping up frequency or volume, do it gradually to avoid shocking the spam filters.
- Use a shared IP address if you don’t have enough consistent volume to maintain a good reputation on a dedicated IP.
- If you’re in a seasonal business, make an effort to stay in touch during the off-season so you’re not going from 0 to 60 come peak time.
Consistency is key for staying in filters’ good graces. Treat your email marketing like a long-term relationship — show up regularly, build positive associations, and reap the rewards over time.
7. Your List is Full of Spam Traps and Dead Ends (and Purchased Lists)
Spam traps and invalid addresses are lurking dangers in purchased lists. Even if you think the seller’s claims of a “clean list” sound promising, sending to people who never opted in can tank your engagement and trigger spam filters. Worse, some addresses are specifically designed as spam traps, and hitting just one can severely damage your sender reputation.
How to Build a Clean List the Right Way:
- Offer Incentives: Use lead magnets like free resources, downloadable eBooks, or exclusive offers to entice people to sign up.
- Promote Your List: Place sign-up forms prominently on your website, blog, and social media channels. Make sure the value of subscribing is clear.
- Leverage Social Proof: Show how many people have already joined your list or share testimonials from satisfied subscribers to build trust.
- Host Events or Contests: Run webinars or giveaways to engage potential subscribers and collect their email addresses.
A growing, engaged email list built with these methods will ensure your emails reach the inbox and maintain your sender reputation over time.
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8. Low Engagement (and the Inactive Subscribers Making It Worse)
Engagement metrics like open rates and clicks are vital signs for email health. When subscribers don’t interact with your emails, spam filters take notice. If a chunk of your list is inactive, it’s not just dead weight — it’s actively dragging down your deliverability.
How to Revive and Maintain Subscriber Engagement:
- Re-engage inactive subscribers with personalized campaigns designed to win them back.
- Segment your list to ensure you’re sending the right content to the right people.
- Remove users who haven’t engaged in 6+ months — it may seem painful, but it helps keep your list healthy.
- Focus on value-driven content that encourages opens and clicks every time.
Active subscribers signal to email providers that your messages are wanted, which helps everyone stay out of spam purgatory.
9. You’re Making Subscribers Jump Through Hoops to Opt Out
Burying your unsubscribe link in a microscopic footer or making people log in to manage preferences is a major no-no. It’s not just bad for the user experience — it’s also illegal under laws like CAN-SPAM.
Keep Unsubscribes Simple:
- Put your unsubscribe link in an easy-to-find place (the footer of every email is a good default).
- Let people opt out in one or two clicks — no password required.
- Process unsubscribe requests immediately (within 10 business days is the legal requirement; faster is better).
- Offer a one-click unsubscribe option and a way to manage preferences.
10. Your Metrics are a Mystery
“What gets measured gets managed.” If you’re not regularly tracking key email metrics like opens, clicks, unsubscribes, and spam complaints, you’re flying blind.
What to Track:
| Metric | Goal | What It Tells You |
|---|---|---|
| Open rate | 20-30% | Subject line effectiveness; list hygiene |
| Click rate | 2-5% | Email content/offer relevance; CTA effectiveness |
| Unsubscribe rate | <0.5% | Content relevance; email frequency |
| Spam complaint rate | <0.1% | Deliverability; subscriber satisfaction |
Tools like Google Analytics and your email service provider’s reporting can give you the data you need to diagnose deliverability issues.
11. Your Reputation is on the Line (Literally)
Your sender reputation acts as your email credit score, shaping how inbox providers perceive you. Spam complaints, bounces, or hitting spam traps damage this score. Even if you’re doing everything else right, a poor reputation can sink your deliverability.
How to Strengthen Your Reputation:
- Use double opt-in so only engaged users join your list.
- Monitor your domain and IP reputation with tools like Google Postmaster or Sender Score.
- Gradually warm up new IPs by starting with smaller campaigns.
- Keep an eye on blacklists and resolve issues quickly if flagged.
A solid sender reputation is earned through consistent best practices — don’t let one mistake undo months of effort.
12. You’re Violating Anti-Spam Laws
The CAN-SPAM Act sets some pretty clear ground rules for commercial email, like including your physical address and making it easy to unsubscribe. Violating these rules can result in steep fines — up to $46,517 per email!
Common CAN-SPAM Mistakes:
- No physical address in email (P.O. boxes are allowed).
- No clear way to unsubscribe.
- Not honoring unsubscribe requests within 10 business days.
- Misleading “From,” “To,” or “Reply-To” information.
- Deceptive subject lines.
How to Stay CAN-SPAM Compliant:
- Always include your company name and address in the footer of every email.
- Put a clear unsubscribe link in every email.
- Process unsubscribes immediately, and don’t require log-in to opt-out.
- Use real, accurate “From” and “Reply-To” information for all your emails.
- Be honest and straightforward in your subject lines — no bait-and-switch.
13. Your Email Content is a Snooze Fest
Here’s a harsh truth: your subscribers don’t want to read your emails. They want to read emails that interest them, entertain them, educate them, or help them solve a problem.
If you’re just churning out generic, me-focused content, your engagement will drop and your emails will start looking like spam to inbox providers.
How to Level Up Your Content:
- Talk about your subscribers more than yourself. Use “you” more than “we.”
- Tell stories, paint pictures, and use analogies to make your content come alive.
- Write like a human, not a corporate robot. Imagine you’re writing to a friend.
- Use humor, personality, and emotion to spice up dry topics.
- Make every email serve a clear purpose (educate, entertain, motivate, etc.).
At the end of the day, if you want people to look forward to your emails, you need to pack them with content that’s actually worth reading.
14. You’re Leaving Money on the Table with Bad CTAs
Your calls-to-action (CTAs) are the moneymakers in your email — literally. They’re what turn subscribers into clickers, and clickers into buyers. But if your CTAs are weak, confusing, or nonexistent, you’ll leave money on the table and your engagement rates will suffer.
Tips for Crafting Compelling CTAs:
- Use action-oriented words (“get,” “try,” “reserve”) to inspire clicks.
- Weave benefits into your CTA (“Get your free ebook,” not just “Download now”).
- Make your CTA stand out with a button or bold text.
- Test different wording and placement to see what gets the most clicks.
- Use urgency and scarcity to encourage immediate action (“Only 3 spots left!”).
The goal of every email is to get the reader to take a specific next step. Guide them to that next step with a clear, benefit-driven CTA.
15. You Only Reach Out When You Want Something
If the only time your subscribers hear from you is when you’re trying to sell them something, you’ll quickly wear out your welcome. Inbox providers will notice the low engagement and start filtering your emails to the spam folder.
How to Nurture Subscribers Between Promotions:
- Send a variety of email types (newsletters, announcements, etc.) to keep people engaged.
- Share helpful resources like blog posts, ebooks, and webinars.
- Run a contest or giveaway to re-engage subscribers and generate buzz.
- Send a survey to gather feedback and show subscribers you value their input.
The key is to mix up your email content so your subscribers don’t feel like you’re always selling to them. Be a helpful resource, not just a virtual salesperson.
16. You’re Not Optimizing for Mobile
Over 50% of emails are opened on mobile devices. If your emails aren’t optimized for small screens, you’ll frustrate subscribers and tank your engagement rates. Inbox providers will take notice and start sending you to the spam folder.
Mobile Optimization Musts:
- Use a responsive email template that adapts to any screen size.
- Keep your subject lines short (aim for 30 characters or less).
- Use a single-column layout for easy scrolling.
- Make your CTA buttons touch-friendly (at least 44×44 pixels).
- Use a legible font size (at least 14 pixels for body text).
Think of your emails as mini web pages. They need to look great and function well on any device. Ignore mobile optimization at your own risk.
17. You’re Including Too Many Links
Including too many links in your emails is a major red flag for spam filters. It’s a common tactic used by spammers to lure people to dodgy websites.
How to Keep Your Link Count in Check:
- Stick to a few relevant links per email (1-3 is a good rule of thumb).
- Avoid using URL shorteners, as they can look suspicious to spam filters. If you must use them, use a branded shortener.
- Link to reputable websites that are relevant to your content and audience.
- Use descriptive anchor text for your links, not generic phrases like “click here.”
Remember, the goal is to provide value with your links, not overwhelm your subscribers with a barrage of links.
18. You’re Going Overboard with Attachments
Attachments, especially large ones, can trigger spam filters and annoy your subscribers. They can also slow down your email’s load time and eat up your subscribers’ mobile data.
How to Handle Attachments:
- Avoid including attachments unless absolutely necessary.
- If you must include an attachment, keep it small (under 1MB) and relevant.
- Consider hosting the file on your website and linking to it instead of attaching it directly.
- Always virus-scan attachments before sending to protect your subscribers.
In most cases, it’s better to link to a resource than to attach it directly. Your subscribers (and their inboxes) will thank you.
19. Improperly Configured HTML
Broken HTML, missing tags, and sloppy code can make your emails look unprofessional and trigger spam filters. It can also cause rendering issues across different email clients and devices.
How to Keep Your HTML Clean:
- Use a reputable email template or have a developer build your template.
- Validate your HTML before sending to catch any errors or broken tags.
- Keep your code clean and concise. Avoid excessive inline styles or complex layouts.
- Test your emails across multiple clients and devices to ensure consistent rendering.
Remember, your email’s HTML is the foundation of your design. If it’s a mess, your whole email will suffer.
Conclusion
Phew, we’ve covered a lot of ground! Improving your email deliverability can feel like a daunting task, but it all comes down to one thing: Respecting your subscribers.
Every tactic we’ve covered here — from setting up authentication to optimizing your content for engagement — is really about building trust with your subscribers and the inbox providers.
So as you work to implement these best practices, always keep your subscriber’s experience front and center. Ask yourself:
- Would I want to receive this email?
- Does it provide real value or is it just cluttering up the inbox?
- Is it relevant and interesting to the people I’m sending it to?
If you can answer “yes” to those questions, you’re well on your way to the inbox.
To recap:
- Authenticate your emails to prove you’re a legitimate sender
- Maintain a positive sending reputation by keeping your list clean and engaged
- Write compelling, spam-free subject lines that accurately reflect your content
- Provide real value to your subscribers with every email
- Make your emails mobile-friendly and easy to interact with
- Stay compliant with email laws and regulations
- Avoid spam trigger words and phrases in your subject lines and body copy
Even if you do everything right, you might still see the occasional spam folder placement. It happens to the best of us. But if you stay focused on sending valuable, relevant content to people who actually want to hear from you, you’ll win in the long run.