Why are my emails going to spam? Common causes and how to fix them

Spam folders exist for a reason. Every day, billions of unwanted emails are sent around the world. Email providers like Gmail, Outlook and Yahoo want to protect their users from that. They do this with advanced filters that evaluate every incoming email. If there's even a small doubt about a message, it ends up in the spam folder.

The frustrating part is that this also happens to emails you actually want to receive. Transactional emails like an order confirmation, a password reset or an invoice. As a sender, you think everything is set up correctly, yet your email doesn't reach the recipient. Or worse: it's buried in the spam folder.

In this article, we explain how spam filters work, why your emails might be going to spam and what you can do about it. We walk through the most common causes so you know exactly where to look.

What is spam email and how do spam filters work?

Spam is any email the recipient didn't ask for. Think of phishing attempts, fake offers or newsletters you never signed up for. But spam isn't always malicious. Sometimes it's a newsletter you once subscribed to but forgot about, or an automated notification you no longer find relevant.

Email providers don't always make that distinction. For them, it comes down to one question: does the recipient want this email? To figure that out, they use spam filters that run every incoming email through multiple checks.

How do spam filters work?

Spam filters used to rely on simple word lists. Did your subject line contain the word "free"? Spam. Those days are over. Modern spam filters from Gmail, Outlook and Yahoo combine multiple techniques to determine whether an email is wanted.

How these filters work exactly is something providers keep secret on purpose. Once the exact rules are known, spammers will exploit them to bypass the filters. What we do know are the general principles every spam filter looks at.

Authentication

The first thing that gets checked is whether the sender is who they claim to be. This happens through SPF and DKIM. SPF checks whether the sending server is authorized to send emails on behalf of your domain. DKIM verifies with a digital signature that the content wasn't altered in transit. On top of that, there's DMARC. DMARC isn't authentication by itself, but it determines what should happen when SPF or DKIM fail. Major email providers like Gmail, Yahoo and Outlook now require all three for bulk senders. Missing one? Then there's a good chance your email gets rejected or lands in spam.

Email domain reputation

Every sender builds a sending reputation with email providers. Your domain and the IP address you send from each receive a score. That score is based on how recipients interact with your emails and how you behave as a sender. A poor reputation means your emails are more likely to end up in the spam folder.

Recipient engagement

Email providers look at how recipients interact with your emails. Are they opened? Do people click through? Or are they deleted without being read? This is called engagement and it's measured per individual recipient. The same email can land in one person's inbox and another person's spam folder, purely based on how they've interacted with your emails before.

Content

The content of your email is also evaluated. Spam filters analyze the layout, the links, the ratio between text and images, and whether the HTML code is properly structured. Providers increasingly use machine learning to recognize patterns from millions of previously flagged spam messages.

Every provider filters differently

Gmail, Outlook and Yahoo don't apply the same rules. Gmail weighs your domain reputation most heavily. If your domain has a poor reputation with Gmail, a good IP address won't save you. Outlook looks at a combination of factors and uses its own scoring system to determine how suspicious an email is. Yahoo filters most aggressively at the IP level, meaning the reputation of your sending IP carries extra weight there.

The result: your email arrives perfectly at one provider and lands in spam at another. That can make it tricky to figure out what's going wrong.

8 reasons why your emails are going to spam

You've set everything up, sent your first email and it still goes to spam. Frustrating. In most cases, it's one of the following reasons.

1. DNS authentication is not set up correctly

This is by far the most common cause. To send emails from your own domain, you need DNS records like SPF, DKIM and a bounce record. These records prove to receiving mail servers that you've authorized emails to be sent from your domain.

If one of these records is missing or contains a typo, the receiving mail server can't verify your email. The result: your email is treated as suspicious and ends up in the spam folder.

With Lettermint, you don't have to worry about this. We continuously check whether your DNS authentication is correctly configured. You can only start sending once everything is in order.

2. Poor email domain reputation

Email providers like Gmail and Outlook assign a reputation score to your domain. That score is based on how recipients respond to your emails. Do they open your messages? Or do they mark them as spam? The more negative signals, the lower your reputation.

It's all about the ratio between the number of emails sent and the number of spam complaints. If you send a hundred emails and five of them get marked as spam, that's 5%. For providers, that's a serious signal. And the annoying part is that a poor reputation doesn't only affect your marketing emails. Your transactional emails) like password resets and order confirmations will also land in spam more often.

Recovery takes time. Weeks, sometimes months. Consider using an email subdomain to separate your transactional emails from your broadcast emails. That way, you protect your most important emails if something goes wrong with a campaign.

3. New domain without sending history

A new domain has no reputation with email providers. Gmail, Outlook and Yahoo simply don't know who you are. They have no data to determine whether you're a legitimate sender or a spammer. As a result, your emails are treated with extra suspicion and land in spam more quickly.

This is similar to a credit history. Without a track record, you're not automatically trusted. You need to build that reputation by gradually sending more emails and collecting positive signals. Think of recipients opening your emails, clicking through and replying.

The mistake many senders make is sending large volumes right away from a new domain. That's a red flag for providers. Start small and build your volume step by step over a few weeks. This gets providers used to your sending pattern and builds trust.

4. Spam-triggering content

The content of your email also plays a role. Spam filters scan your subject line, body text, links and attachments. Certain patterns trigger these filters faster than you'd think.

Think of words like FREE, CLICK HERE, LAST CHANCE or multiple exclamation marks in a row. A subject line written entirely in capitals is also a classic spam trigger. But it's not just about text. Links matter too. URL shorteners like bit.ly or links to Google Forms are seen as suspicious by spam filters. Always use direct links to your own domain. Especially during busy periods like Black Friday and holidays, spam filters become extra strict on these patterns.

At Lettermint, we built Spam Insights for exactly this. This tool analyzes your email and shows a spam score. You'll see right away which parts are causing problems before you send the email to real recipients. You can also test for free using our test email addresses to check if everything arrives correctly.

5. Too many spam complaints and hard bounces

Every time someone marks your email as spam, that counts as a spam complaint. Email providers track this precisely. But hard bounces count too. A hard bounce means your email permanently can't be delivered, for example because the email address doesn't exist. Too many bounces signal to email providers that your list isn't well maintained. And that's exactly what spammers do.

If too many complaints or bounces come in relative to the number of emails you've sent, your reputation drops and your next emails are more likely to land in spam. As we explained in cause 2, it's all about that ratio. The rule of thumb: keep your spam complaint rate below 0.3% and your email bounce rate below 2%.

This can escalate quickly. Say you send a newsletter to an old list of contacts you haven't emailed in months. Some addresses no longer exist and hard bounce. Others don't recognize you and mark your email as spam. That doesn't just affect your newsletters, but also your transactional emails.

An unsubscribe link is therefore required. When recipients can easily unsubscribe, they'll do that instead of marking your email as spam. At Lettermint, we automatically add an unsubscribe link to broadcast emails. That protects your reputation.

6. Poorly structured emails

The technical structure of your email plays a bigger role than you might think. Spam filters don't just look at what's in your email, but also at how it's built.

A common mistake is sending an email that only contains HTML. A well-structured email always includes two versions: an HTML version and a plain-text version. This is called a multipart email. The HTML version is the formatted variant most recipients see. The plain-text version is the bare text version without formatting, as a fallback. If the plain-text version is missing, spam filters may see that as a reason to distrust your email. Spammers tend to skip it.

But the HTML itself can also cause problems. Messy or broken HTML code, an email that consists only of images without text, or a poor ratio between images and text are all signals that spam filters pick up on. The cleaner and simpler your code, the better.

For transactional emails, this is especially important. A password reset or order confirmation doesn't need to look like a marketing newsletter. Keep it simple. The more your email looks like a personal message, the better spam filters handle it.

7. Low recipient engagement

Email providers like Gmail and Outlook don't just look at who you are, but also at how recipients respond to your emails. Are your emails being opened? Do people click? Or do they sit untouched in the inbox?

When a large portion of your recipients consistently ignores your emails, that's a signal to email providers that your messages aren't wanted. The result: your emails get moved to spam more often. And as we discussed in cause 2, that directly impacts your sending reputation.

This mainly affects broadcast emails. If you send a newsletter to thousands of recipients and only a small percentage opens or clicks, that works against you. Providers conclude that your emails add little value. If you then send a transactional email from the same domain, that can be affected too.

The solution: only send to recipients who are engaged. Remove inactive contacts from your mailing list regularly. A smaller list with high engagement is better than a large list where nobody responds.

8. Poor IP reputation

Besides your domain reputation, the reputation of the IP address you send from also matters. Email providers look at the behavior that has been associated with that IP address in the past. Has spam been sent from it before? Then your emails will also be treated with suspicion.

This is a common issue when you send emails from a VPS or your own server. Many cheap hosting providers recycle IP addresses that were previously abused by spammers. You get assigned an IP address with a tainted history without even knowing it. Your authentication is correct, your content is fine, but your emails still land in spam.

Building a good reputation on such an IP address takes weeks of consistent, careful sending. And even then it's not guaranteed, because some IP addresses are on blacklists that are hard to get removed from.

At Lettermint, a healthy IP is the foundation of everything. That's why we verify every new user before they can send. We monitor our IP addresses 24/7 and maintain a zero-tolerance policy against spam. Senders who don't follow the rules are immediately removed from our platform. This protects the deliverability of all our customers.

What does Lettermint do to help?

Behind the scenes, we do a lot to make sure your emails reach the inbox. From technical requirements to active monitoring: here's what we take care of.

• You can't send without DNS records. Before you can send, SPF, DKIM and DMARC must be correctly configured for your domain. Without these records, you simply can't send through Lettermint. This way, email providers know your emails are legitimate.
• We scan every email before it's sent. Every email is automatically checked for known spam triggers before it leaves our servers. This happens entirely on our own infrastructure, without external services or AI. If a message scores too high, we block it to protect your reputation.
• Broadcast emails automatically get an unsubscribe link. For broadcast emails, we automatically add an unsubscribe link, along with the required List-Unsubscribe headers. This is required by providers like Gmail and Yahoo. If you already send these headers yourself, we'll use yours.
• You can see your spam score right away. After sending, you'll see a spam score per email in your dashboard. This gives you immediate insight into what you can improve. Pro users get a detailed breakdown of the exact causes through Spam Insights.
• We verify every new user. Every new user is verified before they can start sending. This keeps bad actors out and protects the platform for all our customers.
• Large campaigns are spread out automatically. For broadcast emails, we automatically distribute large volumes over time. This prevents a sudden spike in sending volume from triggering receiving email providers.
• We keep a close eye on our IP reputation. We monitor our IP addresses continuously. Unusual patterns or spikes in sending volume are immediately detected and addressed.
• We reach out when there's a problem. If we receive spam complaints about your emails or notice an unusually high number of hard bounces, we contact you and help you prevent it in the future.
• Hard bounces can't be re-sent. Email addresses that produce a hard bounce are automatically added to your suppressions list. You can't send to these addresses again. This protects your sending reputation against repeated bounces.

Email deliverability checklist: prevent emails from going to spam

You've read everything above but want to quickly check if your emails meet the most important requirements? Run through this checklist. We focus on the things you can actually control: technical settings, the content of your emails and how you handle your recipients.

• DNS records complete? Check that SPF, DKIM and DMARC are correctly configured for your sending domain. Sending through Lettermint? These records are provided when you add your domain.
• Are you sending from a dedicated (sub)domain? Use a separate subdomain for transactional emails to protect your main domain. Also sending newsletters? Consider a separate subdomain per email type.
• How's your domain reputation? Keep your bounce and spam rates low. A high bounce rate or too many spam complaints damage your reputation with email providers.
• Does your email contain spam triggers? Avoid words like "free", "click here" or "urgent". Don't write your subject line in all caps and avoid links to external domains.
• Prevent spam complaints. Only send to people who expect your emails and keep your complaint rate below 0.3%. For broadcast emails, Lettermint automatically adds an unsubscribe link.
• Is your engagement healthy? Remove inactive recipients and keep your lists clean. Low open rates are a signal for email providers.
• Consider corporate email filters. Business recipients may have additional content filters that block your emails. Ask recipients to whitelist your domain if needed.
• Test your emails before sending? Always send a test email first to check that everything looks good and is delivered correctly.

Conclusion

Everything starts with a healthy sending reputation. But that reputation isn't something you set up once and forget about. It's an ongoing process where every email you send affects how providers evaluate your next message. Building a good reputation takes time. Damaging it can happen in a few hours.

The most important factor? Make sure recipients want to receive your emails. Every message that's perceived as unwanted is a potential spam complaint. And every complaint counts in how providers view you. Send relevant emails, at the right time, to the right people.

Only send emails you'd want to receive yourself.

Also pay attention to how your emails are structured. Use a good balance between text and images and always include a plain-text version. Avoid spam-triggering words, excessive formatting and links to external domains. The cleaner and more relevant your email, the better providers will evaluate it.

At Lettermint, we do everything we can to get your emails into the inbox. From mandatory authentication to spam filters and active monitoring. Have a question, running into something or want feedback on your emails? Get in touch and we'll be happy to help.