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• Why Your Cold Emails Land in Spam (Even When You Follow the Rules)

Why Your Cold Emails Land in Spam (Even When You Follow the Rules)

Sujan Patel

Sujan Patel is the founder of Mailshake, a sales engagement software used by 38,000 sales and marketing professionals. He has over 15 years of marketing experience and has led the digital marketing strategy for companies like Salesforce, Mint, Intuit and many other Fortune 500 caliber companies.

Read more articles by Sujan Patel →

• October 28, 2025

You’ve done everything right: you personalized the copy, avoided all shady and spammy phrases, and kept your list clean. But when you hit send, your reply rate still stays at 0%. Sound familiar? The problem isn’t your copy; it’s what’s happening behind the scenes.

The way email inboxes work has changed. Filters are no longer just rule-based; they use AI to learn the behavior of your cold emails. They analyze sending patterns, engagement velocity, and content at scale to identify mass outreach, even if it’s well-disguised.

And if they find your cold email campaign even slightly suspicious, they’ll often “shadow ban” it. This will leave you with perfect deliverability reports while your emails never reach your human prospects.

Why does this happen? In this article, we’ll discuss 11 reasons why your emails land in spam and what you can do about it.

1. AI-Powered Spam Detection

Gmail and Outlook now use advanced AI models that keep up with the rules and recognize patterns of cold outreach to better implement those rules. This means they can easily detect generic, mass email behavior patterns that human reviewers might miss.

Even senders with a perfect technical setup and clean content aren’t safe. If your sending patterns match spam behavior, the AI will flag you regardless of your email’s content. This includes:

• Sending at robotic intervals
• Maintaining high volume with low reply rates
• Using templates that generate similar engagement signals to known spam campaigns

The only way to avoid getting flagged by AI is to follow all cold email best practices and not act like a spammer. This means you need to complete a proper domain warmup before campaigning and focus on generating replies rather than just sending.

2. Sender Reputation Issues

Mailbox providers like Gmail or Outlook constantly score your domain’s trustworthiness based on your sending history, user complaints, and bounce rates. A low score means your emails will be filtered.

Your emails may be landing in spam if you have:

• A new domain with no sending history (providers filter unknown senders by default)
• A previously abused domain with a negative reputation from past spam activity

Here’s how to fix it:

• Use a dedicated domain for cold outreach. This protects your primary company domain while you build a reputation.
• Start with a clean domain. Either acquire new domains or thoroughly vet existing ones to confirm no negative history.
• Build a reputation gradually. Begin with low sending volumes and increase slowly to establish trust with providers.

3. Technical Authentication Problems

SPF, DKIM, and DMARC are mandatory protocols that authenticate your emails. They prove to mailbox providers that you are an authorized sender.

If they’re missing or misconfigured, this will make it seem like your emails are coming from an unauthorized sender. Email providers will filter these emails because they can’t verify the sender, which is a major red flag for spam.

Here’s how to fix this:

• Verify your records using a tool like MXToolbox.
• Implement all three protocols: SPF, DKIM, and DMARC. Having just one or two is insufficient.
• Re-check your configuration every time you change your email service provider.

4. Insufficient Warmup

Warm-up is the process of building a positive sending history. It helps mailbox providers realize that you’re an actual human sender, not a bot or spammer.

But if you’re sending high volumes of emails from a “cold” domain with no history, this is a major red flag for spam filters. Email providers see this sudden activity as suspicious and will filter your emails, which is why you may end up in spam..

Here’s what you can do to fix this:

• Commit to a full 2-4 week warm-up period before starting any cold email campaigns.
• Use a dedicated warm-up service, like the one built into Mailshake, designed to generate real, two-way email conversations. Replies are what help you build a positive reputation.
• Begin with very low volumes — 5-10 emails per day — and only gradually increase the sending volume over time.

5. Poor List Hygiene

List hygiene is the quality of your email list. If it contains invalid, non-existent, or catch-all addresses, this will lead to hard bounces that can damage your sender’s reputation.

You may also hit hidden spam traps, especially if you’re using old or purchased lists. Internet service providers (ISPs) plant these traps to identify senders who don’t follow proper list acquisition practices.

Here’s how to fix it:

• Verify every email address using a reliable verification service before sending. Never skip this step.
• Remove all role addresses (info@, admin@, sales@) from cold outreach. They rarely engage and often report spam.
• Maintain a hard bounce rate below 0.5% across all campaigns. Regularly clean your lists to achieve this.

6. Email Content Triggers

Modern spam filters use AI to analyze your email’s entire structure and language patterns, not just individual words. They’re trained to detect mass outreach behavior, even when you use personalization.

Overly sales-focused language, multiple links, and HTML-heavy templates create patterns that match known spam campaigns. The AI recognizes these structural red flags and filters your emails before they ever reach the inbox.

Here’s how to fix it:

• Remove salesy words like “free,” “guarantee,” “opportunity,” and “solution” from your copy.
• Never include links in your first cold email. Focus the call-to-action on generating a reply.
• Write in plain text format, avoid HTML templates, and keep your message concise and direct.

7. Lack of Personalization

Personalization doesn’t just mean mentioning your recipient’s first name and calling it a day. It requires you to make contextual references to their company, role, or recent activities to show you’ve actually researched them.

When you don’t personalize your emails enough, like mentioning something about the recipient or their company in your email body, you might see low engagement in the form of few replies or quick deletions.

This lack of positive response will signal to AI filters that your messages are unwanted or don’t contain valuable information, which may cause them to shadowban you. This will damage your sender’s reputation and future deliverability.

Here’s how to fix this:

• Include 3-4 specific personalization points in your cold email’s body to show that you performed actual research.
• Use tools like Mailshake’s SHAKEspeare AI to quickly generate personalized, contextual references based on a prospect’s company and role.
• Create hyper-segmented lists to ensure message context matches each recipient’s situation.
• Write conversational, human-to-human text, something that you’d say to a colleague or people you know.

8. High Sending Volume per Inbox

Each individual email address you send from has its own reputation and daily sending limits with providers like Google and Microsoft.

Sending 50+ emails per day from a single account doesn’t resemble normal human behavior. This activity pattern immediately triggers spam filters and internal rate limits, regardless of your email content.

How you can fix it:

• Follow the 20/3 rule: send no more than 20 emails daily per inbox, using a maximum of three inboxes per domain. (Mailshake’s multiple inboxes are key here)
• Distribute your sending volume across multiple dedicated domains and email accounts.
• Maintain these limits consistently rather than sending in large bursts.

9. Domain and Email Architecture Mistakes

When your domain and email accounts aren’t set up correctly, providers will flag your emails before they’re even delivered.

For instance, if you use email aliases, like name+marketing@company.com, instead of separate user accounts, it’ll look like you’re trying to hide your sending activity. Providers see this as suspicious behavior.

Similarly, if you forward emails between domains without proper configuration, it’ll break your authentication and make your emails look fraudulent, which may land you in spam. Here’s how to fix this:

• Create unique user accounts for each sender in Google Workspace.
• Never rely on aliases for your primary sending accounts.
• Make sure your domain has proper DNS configuration, even without a website.

10. Lack of Engagement Signals

Engagement is the most important signal for mailbox providers. Because when recipients reply to your emails, move them to their inbox, or consistently open them, providers learn that people want your messages.

But if your emails get ignored, immediately deleted, or marked as spam, you’re training the AI that your content is unwanted. This tells providers to filter your future emails before they ever reach anyone’s inbox.

Here’s how you may be able to fix this:

• Focus on generating replies above all else; aim for at least a 15% reply rate.
• Write subject lines and opening lines that make people want to respond.
• Pause your campaign and fix what’s not working if an email isn’t getting replies.

11. Volume Management and Cadence Errors

How you distribute your emails throughout the day matters just as much as your total daily volume because spam filters analyze your sending patterns, not just your numbers.

When you send all your emails in a tight window (like 50 emails in one hour), you seem like a bot. But if your volume fluctuates wildly (sending 100 emails one day, then five the next), you resemble a spammer who only activates with fresh lists. Both patterns trigger filtering.

Here’s how to fix this:

• Use your email software’s “scheduling” feature to spread sends evenly throughout business hours in your recipient’s time zones.
• Match sending patterns with human behavior; real people don’t send 50 emails at 2 AM.
• Maintain consistent daily volume and avoid sending more than 20-30% above your established average, especially during warm-up.
• During warm-up, increase volume gradually (no more than 10-15% daily increases).

Final Thoughts

Deliverability isn’t a single setting you can fix. Instead, it’s an interconnected system where failure in any one of the 11 areas we covered can sink your entire campaign. But even perfect deliverability means nothing if your emails don’t generate replies.

That’s where engagement comes full circle. When you combine proper technical setup with sequences that actually engage prospects, you create a cycle where better emails generate more replies, which improves your sender reputation, which gets more emails to the inbox.

Mailshake handles this complete process. Our platform manages warm-up sequencing, maintains optimal sending volumes, and provides the infrastructure you need for consistent deliverability through:

• Automated follow-up sequences that keep conversations moving
• SHAKEspeare AI that helps you create personalized, engaging copy
• A/B testing tools to optimize your highest-performing messages

Want to focus on creating outreach campaigns that get replies instead of technical troubleshooting? Book a demo today to see how Mailshake can help you connect with prospects and close deals.