Email deliverability and spam prevention: why your emails aren’t getting delivered and how to fix it

For your emails to have an effect, the users must receive them first. However, it’s not uncommon to see your emails ending up in the spam folder, and have no idea why this happens. We are here to help with that.

In this guide, we will review the common causes of emails ending up in spam and how you can fix each one to improve your chances of landing in the inbox.

The problem

When you (or your app) send an email, the following things happen:

1. First, the email is composed either by a human or a service.
2. The email is sent to the outgoing SMTP server, which communicates with a DNS server. If the DNS server cannot locate the recipient, you get a “mail undelivered” message.
3. If the recipient is located, the email is passed to the recipient’s email server, which decides where the email belongs—whether in their inbox, spam folder, or not delivered to them at all.

Several variables can affect the result of the last step, and not deliver your emails or put them in the spam folder. Some of them are:

• Poor sender reputation: Each Email Service Provider (ESP) assigns a score to each sender—not unlike how search engines rank webpages. Your sender’s reputation is based mainly on how your audience engages with your emails. Other factors also get in the mix, like the content’s quality and the frequency of the messages. A poor sender reputation will result in your emails being in the spam folder.
• Poor domain reputation: Domain reputation measures how trustworthy the domain that sends the emails is. ESPs assess the history of the emails your domain has sent to determine if it’s legitimate or spam. Factors contributing to that are engagement rates, adherence to best practices, and whether people make spam complaints against you. The reputation of a domain stays the same even if you change the IP address. You can check your domain’s reputation using tools like Google Postmaster, Yahoo Postmaster, and Microsoft SNDS.
• High bounce rates: Sending emails to outdated or invalid addresses leads to high bounce rates, which is a surefire way to get your emails flagged as spam.
• Spikes in sending volume: Sudden spikes are a huge red flag for providers, as they make it look like you are spamming people.
• Lack of engagement: If people are not engaging with your emails, providers will notice and assume that your messages are not valuable and should be put in the spam folder.

There are more reasons that can contribute to your emails not getting delivered. Things like not including a clear unsubscribe link, using spam words like “buy now”, or using all caps.

It’s important to understand the root of the problem so you can fix it. There are many tools out there that you can use to evaluate your reputation, check if you are in blacklists, or check your content’s format.

The solution

There are some common best practices that businesses follow to ensure the biggest deliverability ratios possible. In this section, we will go through the most important ones.

Warm up the domain

If your sender’s reputation is bad, there is nothing you can do content-wise to improve your deliverability. You must show the providers that you are trustworthy.

This also applies when you are sending from a new domain. You are the new kid on the block and servers don’t know whether they should trust you or not. And when you start sending a large volume of emails things get even worse.

There are two ways to improve your reputation (or build it up for new domains):

• Warm up your domain using one of the tools available online, like mailwarm. These services raise your sender's reputation by positively interacting with your emails. They create a steady flow of positive signals to email providers by opening your messages, replying, and even pulling them out of the spam folder. As a result, your reputation goes up and you don't end up as easily in the spam folder.
• Alternatively, you can do this manually. Send emails to your team and ask them to interact with them in a positive way, like replying or marking them as “Not spam”. This approach works, but it is time-consuming.

Set up properly your authentication records

Email providers can’t verify that your emails are legitimate without proper authentication, which makes it easier for your messages to be flagged as spam.

There are three protocols:

• SPF (Sender Policy Framework): Verifies that the sending server is authorized to send emails on behalf of your domain.
• DKIM (Domain Keys Identified Email): Adds a digital signature to your emails to verify that they haven’t been altered during transit.
• DMARC (Domain-based Message Authentication, Reporting, and Conformance): Provides instructions for how to handle emails that fail SPF or DKIM checks and offers reporting on email authentication results.

Setting up SPF and DKIM is mandatory to prevent emails from going to spam. DMARC is not mandatory, but it’s a good idea to set it up either way, since it enhances the overall security of email communications, offering a comprehensive shield against unauthorized use of a domain. Implementing all three protocols shows email providers that your messages are legitimate.

Use providers with a good reputation

Google Workspace and Office 365 are the top email service providers whose messages make it into inboxes (which makes sense since they own most of the email market). Using them gives you a good chance of not ending up in the spam folder.

Write email content that passes the spam filters

Here are some industry best practices to keep in mind when you are writing email content:

• Don’t always send the same email. Personalize it for each user. This plays a strong role in deliverability.
• Avoid spam words, aggressive punctuation, and capital letters.
• Include an easy-to-spot unsubscribe link in your marketing emails. If people cannot unsubscribe, they will hit that “Mark as spam” option to get rid of your emails.
• Balance text and images. Too many images make an email look suspicious to spam filters.
• Avoid using URL shorteners. It’s a common practice of spammers to hide the nature of the URLs they send.
• Run your emails through deliverability and spam tests to catch any potential issues.

Disable tracking

Spam filters are sensitive to tracking pixels, flagging them as potential spam. Without these tracking elements, emails may bypass these filters more effectively.

Validate your email list

Make sure your emails don’t bounce. High bounce rates damage your sender's reputation and lower your deliverability over time. Use a trusted list validation tool to clean your database regularly.

Use double opt-in

Double opt-in means that, after people select to sign up for your email list, they receive a confirmation email they must use to confirm their subscription. This protects you from erroneous signups and bots so your email list stays healthy and your bounce rate is low.

Stay out of blacklists

There are many databases that list IP addresses or domains known to send spam or malicious content. These are called DNS-based Blackhole Lists (DNSBLs) or Real-time Blackhole Lists (RBLs). When email providers filter spam, they consult those lists and either reject or send any emails from senders in the list directly to spam. There are tools online you can use to check if your domain is blacklisted.

Summary

The first step is identifying the root cause of your emails not being delivered. Once you have, then you should take the proper action to fix the issue:

• Poor sender reputation: Warm up your domain, check your authentication protocols, and ensure you send to users who want to receive your emails.
• High bounce rates: Validate your email list.
• Spam complaints: Ensure you send to people who want your emails and make unsubscribing easy.
• Content triggers spam filters: Follow industry best practices when writing content and test emails before sending them to spot invisible issues with your email templates.
• You are on blacklists: Contact each blacklist and request to be removed while fixing the issues that got you on the blacklist to begin with.

If you are doing cold email campaigns, the list of the things that might be going wrong is slightly longer compared to sending emails only to subscribed users. No matter which case applies to you, following some common best practices can go a long way to get your emails right where they belong: to your user’s inboxes.