If you send marketing emails, you probably expect that your subscribers will at least see your email in their inbox. But if you're reading this article, chances are you’re beginning to suspect your subscribers aren’t even seeing your' emails. If you're beginning to wonder "why is my domain email going to spam?," you're in the right place. In this post, we’re going to take a deep dive into how spam filters work and more importantly, how to how to avoid spam filters.

How Much Of The World’s Email Is Spam?

A full 45% of the world’s email in January of 2021 was unsolicited spam. Spam filters provide the vital service of protecting our inboxes from malware or phishing while also making sure essential messages we want to see aren’t drowned out by spam.

Gmail built its brand in part on having the best spam filter that would clean up your inbox without accidentally grabbing important messages.

In the early days, the primary driver from the spam filters data was simply subscribers flagging messages as spam. If you’re brand was flagged too many times, your reputation fell and you’d start landing in the spam folder. As filters have evolved, new technologies like AI and machine learning are playing a bigger role in spam filtering. Google says of machine learning:

“it also helps us personalize our spam protections to each user—what one person considers spam another person might consider an important message (think newsletter subscriptions or regular email notifications from an application).”

Spam filters aren’t going away anytime soon. So as a marketer, your best bet is to try to understand them better. Once you know the basics of how spam filters work, you’ll be able to take steps to avoid spam filters and keep your emails in the inbox where they belong.

3 Types Of Spam Filter’s Your Emails Will Encounter

Spam filters come in several forms. Let’s look at the three types of spam filters that are most likely affecting your email deliverability.

3rd Party/Cloud-Based Spam Filters And Gateway Spam Filters

Securing a company's network, data, digital assets, and communications is a critical priority for organizations large and small. Many companies use gateway, or cloud-based email spam filters to filter and quarantine both inbound and outbound messages that are suspicious.

Gateway spam filters are installed on servers onsite, whereas cloud-based filters run on 3rd party servers. Functionally, they serve a similar purpose, protecting a company's network by securing its digital borders.

Even though Gmail and Microsoft provide a high level of spam and malware protection on their own, cloud-based and gateway spam filters allow network administrators to have an extra level of control and insight into traffic both in and out of their network.

Barracuda is a commonly used hardware-based gateway spam filter. Examples of cloud-based spam filters include Cloudmark and Symantec.

Corporate gateway and 3rd party spam folders present a unique challenge to B2B marketers as they often throttle or quarantine bulk email deliveries.

Desktop spam filters

Desktop spam filters live on a user’s computer and allow for 1 to 1 configuration and personalization. Microsoft SmartScreen, G-Lock SpamCombat are a few examples of desktop-based spam filters.

Email Service Providers Built-In Filters

For B2C and B2B senders, Google, Microsoft, and Yahoo’s built-in spam and inbox sorting technologies are going to be the most common challenge you’ll face.

Let’s take a look at how the three largest inbox service providers (ISP’s) identify spam. First, we’ll look at the factors they use, and then we’ll look at each provider individually to see how they differ.

7 Factors That Determine If Your Emails Go To Spam, Or The Inbox

To understand how email spam filters work, we first need to understand the factors they use to identify spam.

Source IP

If you’re reading this, you have an IP address; it’s your unique identifier online. One of the first factors email service providers will look at when determining if an email is spam is the IP address of the sender.

If a specific IP address has received many complaints in the past, email from that address is more likely to be identified as spam. Sometimes email from an IP with a very poor reputation may not be accepted by the receiving server at all and you’ll receive a bounce notification.

Shared vs dedicated IP address

Because your sending IP address is a known factor in email deliverability, many companies opt to set up a dedicated email sending IP address. This can be a good idea if you have the resources and know-how to manage it properly.

If you haven’t set up a dedicated sending IP address, your emails are most likely being sent through your marketing automation platform’s shared IP. This means your emails are being sent from the same servers as other customers on your marketing automation platform.

Because your IP address is shared, your reputation is shared. If you misbehave, it affects other users and vice versa.

Using a shared IP address does not automatically doom you to poor deliverability. Most marketing automation companies understand the risks of having a poor IP reputation and place limits on senders who are misbehaving. In some cases, we see worse results from senders using poorly managed dedicated IP addresses than from senders using shared IP’s.

HubSpot, for example, will suspend your email marketing ability for having a spam complaint rate of just .1%. It’s also common that email platforms will group users by reputation. If you behave well, your email will likely go out from IP address’s shared with other high-quality senders.

How to check your email IP Reputation

If you have a dedicated sending IP address, you can check its reputation using services like Sender Score, Reputation Authority, or Talos Intelligence. If you’re on a shared IP, the more important factor in determining where your email lands will be your email domain reputation.

Email Sending Domain Reputation

Increasingly, email service providers are looking not only at the originating IP addresses of senders, but also the sending domain and even the individual alias of the sender.

Let’s say emails from @yourcompany.com are usually deleted without being opened, or worse, marked as spam. Emails marked as spam are a strong indicator that your emails are not a high priority for most people. This means there is little risk that they will be missed if the ISP decides to label them as spam.

On the other hand, if emails from your company are clicked, opened, whitelisted, forwarded, replied too, etc., your domain reputation will benefit.

Several of the large ISP’s provide services to check your domain reputation. You can find links to them in the next section of this post.

Spam Traps

One of the quickest ways to damage your IP and domain reputation is by sending email to spam traps. If a user stops using an email account, eventually, the email provider may shut it down. For Yahoo and Gmail accounts, this happens after 270 days.

Once an email account is locked, ESP’s will sometimes recycle the email address and turn it into a spam trap. Sending to spam traps will not trigger a bounce, but senders who email them may be penalized. This is why it's crucial to remove completely inactive addresses from your email list. The risk of accidentally emailing a spam trap is too high.

Email providers sometimes plant fake email addresses around the web in places where bots may scrape them. These addresses can end up on bulk email lists for sale online. Emailing one of these “pristine” spam traps can levy a hefty penalty for the sender and even land you on an email blacklist.


Blacklists are lists of IP addresses belonging to known spammers or people who let spammers use their infrastructure. We've listed a few well-known blacklists below:

-Return Path Reputation Network Blacklist (RNBL)

-Sbl.spamhaus.org (SBL)

-SpamCop (SCBL)

-Passive Spam Block List (PSBL)

Needless to say, you don’t want to end up on one of these lists.

Sending Rate

Sometimes email can fail to land in the inbox simply because too much volume is sent to the same server at once. Gateway filters like Barracuda allow administrators to rate control bulk email deliveries. If you send the same email to too many contacts at a domain that is using rate controls, your emails may not be delivered.

For this reason, throttling or spreading out your email deliveries over time may itself increase your deliverability. One way to accomplish this is by using a tool like Seventh Sense.

With Seventh Sense, your emails will be sent over a window of time with each recipient getting their email at the predicted best time based on their own open and click history. Throttling is a natural part of send time optimization and reduces the chances your emails will be labeled a bulk delivery. In addition, Seventh Sense will automatically help you identify which contacts are risky to email so you can preserve your email deliverability.


Content and IP address used to be the two primary ways ISP’s sorted mail. Using “spammy” language could result in messages being flagged, so marketers maintained lists of words to avoid.

Spam filters are more sophisticated now and most models look for patterns in content rather than specific words or phrases. Apache Spam Assassin describes its platform as using “a wide range of advanced heuristic and statistical analysis tests on email headers and body text including text analysis, Bayesian filtering, DNS blocklists, and collaborative filtering databases.”

So it’s clear that content plays a role in filtering. However, unless you sell Viagra via email, embed malware or link to spammy sites, it’s likely your content’s biggest effect on deliverability is the effect it has on user engagement.

If you send low-quality content, your users are less likely to engage and may even mark your emails as spam.

That said, there are still some guidelines you should follow. Keep a healthy ratio of text-to-images, write clean HTML, and ensure your emails create a good user experience across email clients.

Remember, many email clients block images by default. Make sure your email can still be understood even with images off.


Authentication protocols are one of the many different ways ISP’s verify email senders and prevent hackers and spammers from reaching your inbox.

The three authentication protocols you should be aware of are DKIM, SPF and DMARC. You can read more about them in our post on email domain reputation here.

Emails that don’t pass authentication are more likely to be classified as risky or spam by ISP’s. Invest the time to make sure your marketing emails are properly authenticated.

How Does Gmail’s Spam Filter Work?

1.8 billion people use Gmail every month and over 6 million businesses pay Google to use Gmail as part of G Suite. The chances are good that Gmail-based addresses make up the majority of your email list.

Gmail uses a combination of rule-based filters and machine learning, most recently integrating TensorFlow-based AI into their spam filter. Let’s take a look at some of the factors that go into determining if your marketing emails go to spam.

IP and Domain Reputation

Gmail looks at both the domain and the IP address of senders when determining where to place an email. Gmail recommends setting up proper authentication for your outgoing email to make sure your messages aren’t blocked or filtered.

User Engagement

Gmail’s algorithms appear to strongly weigh individual user actions in the inbox when deciding which emails to send to spam. What may be spam for you are promotions for me, and inboxed for someone else.

Examples of user actions that might be considered include:

  • Messages deleted without being read
  • Messages marked as spam
  • Emails voted as not spam
  • Messages moved to promotions
  • Starred messages
  • Messages forwarded
  • Messages read
  • Messages replied to
  • Spam reports or complaints

To stop your emails from going to spam in Gmail, you’ll want to make sure you’re regularly cleaning your list of subscribers who never engage. You’ll also want to use a different engagement strategy for passive subscribers, or contacts who show lower levels of engagement.

Continuing to use the same email cadence as you use with your most active subscribers will increase the risk that Google will deem future messages as unwanted.


Your email’s header, body, images, and links are all relevant factors in determining if an email is spam, promotional, social or inbox worthy.

Content remains part of the general filtering algorithm, but the level of enforcement appears to now depend more on the sender’s reputation. Avoiding “spammy” words does not appear to be as important as maintaining a high level of subscriber engagement.

Sending History

If using a dedicated IP, new IP addresses should be warmed up.

According to Return Path, Gmail’s default action on new IP addresses is to block them for the first 2-24 hours temporarily. After that, a small number of emails are delivered to the inbox. Another small group is sent to spam to check the recipients’ reactions.

If this initial test results in a high complaint rate, most future emails will be sent to spam. On the other hand, if recipients rescue messages from spam by clicking “This is not spam,” Gmail will deem the address safe for inboxing.

How To STOp Emails From Going To Spam In Gmail

The best way to stop emails from going to spam in Gmail is to follow their list of Sender Guidelines. Item number one on their list is to use Gmail Postmaster Tools.

Gmail’s Postmaster Tools is simple to set up, and gives visibility into your current domain reputation, IP reputation, spam complaint rate, and other deliverability statistics. It can have glitches, but overall it is a handy tool for email marketers.

How Does Outlook Detect Spam?

Microsoft is one of the oldest webmail providers having launched Hotmail.com in 1996. In 2013, Microsoft rebranded Hotmail.com to Outlook.com, which currently has over 400 million active users.

Microsoft pays attention to engagement, complaints and spam traps and other factors similar to other email service providers, but they also rely heavily on their Sender Reputation Network.

Microsoft Sender Reputation Data Network (SRD)

Also known as the Spam Fighters program, Microsoft’s Sender Reputation Data network uses a panel of voters selected randomly from active Outlook users to help train their filters.

Emails received by SRD panel members may be resent to the members with a message asking them to vote whether the original email was “Junk” or “Not junk”. As you would expect, high “Junk” votes will likely lead to your future emails being less likely to make it to the inbox.

SRD may be more reliable than monitoring complaint rates alone. Senders can easily manipulate complaint rates by sending a higher volume of email to reduce the number of complaints.

With SRD, it’s harder to artificially lower your complaint rate by sending a high volume of email.

How To Avoid Spam Filters When Sending Emails To Outlook

As with other providers, following best practices like regularly scrubbing your list of disengaged contacts, authenticating emails, and sending high-quality and relevant content will decrease the odds your emails go to spam in Outlook.

Senders using a dedicated IP address can request access to Microsoft's Smart Network Data Services (SNDS). SNDS is essentially Outlook’s Postmaster tools. It allows marketers to request access to their IP range and see data on subscriber complaints, email volume, spam trap hits, etc.

In addition, senders can enroll in Outlook’s Junk Email Reporting Program (JMRP) feedback loop service to receive copies of messages their recipients have marked as spam. If you are following best practices and still encountering deliverability issues, you can submit a support ticket to Microsoft directly.

How Does Yahoo Filter Spam?

According to their FAQ, Yahoo’s spam filters monitor the following factors:

  • IP address reputation
  • URL reputation
  • Domain reputation
  • Sender reputation
  • Autonomous System Number (ASN) reputation
  • DomainKeys Identified Mail (DKIM) signatures
  • Domain-based Message Authentication Reporting and Conformance (DMARC) authentication

The same best practices you need to inbox with Google and Microsoft will increase deliverability with Yahoo Mail.

How To How To Avoid Spam Filters When Sending To Yahoo

Yahoo offers several guidelines for marketers to improve their deliverability on its Postmasters page. In addition to the standard recommendation of list cleaning, authentication, etc., Yahoo also recommends keeping email volume constant by throttling if necessary.

From Yahoo: “If you send emails at a certain rate and suddenly have a spike of activity, you could get flagged as a compromised sender and marked as spam. Instead, plan your campaign and spread it out over a period of time.”

Yahoo also specifically recommends publishing a reverse DNR record if you have a dedicated sending IP. From Yahoo: “Not having a reverse DNS entry can cause your mailing IP to look like a dynamically-assigned IP instead of a static mail server.”

You can enroll in Yahoo’s Complaint Feedback Loop to receive notifications when users mark your emails as spam or complain. If you continue to have issues with deliverability with Yahoo Mail, you can submit your sending IP address to Yahoo directly for review.

Are You Ready To Optimize Your Email Marketing?

Now that you know how to prevent emails from going to spam, it's time to put what you’ve learned into practice. This article has actionable tips to help you take the steps you’ll need to ensure your emails have the best chance of making it to the inbox every time.

If you're a HubSpot user looking to take your email marketing to the next level, you can audit your HubSpot deliverability with this free tool. You might also consider automating your engagement strategy with Seventh Sense. Seventh Sense works with HubSpot and Marketo to help you optimize your email deliverability and reach customers at the right time. Schedule a demo to learn more.


Ivan LaBianca

Written by Ivan LaBianca

Ivan is the Chief Revenue Officer of Seventh Sense, an optimization system that leverages artificial intelligence to drive better results with email marketing. In his role, Ivan oversees all revenue operations which includes sales, marketing, and customer success. Prior to Seventh Sense, Ivan was an independent marketing consultant and has supported organizations such as The Conservation Fund, 2Stone and Beyond Microgreens. Ivan started his career as a photojournalist, which brought him to countries such as Haiti, India, Nepal, Tunisia, Libya, and others. His photos have been widely published including by outlets such as The Wall Street Journal, Amnesty International, NPR, CNN, MSNBC, and National Geographic Online.