Everything you need to know about email deliverability to make sure that your emails reach the inbox and never land in spam. More than 30 emails key points analyzed, including practical guides and relevant tools to use to maximize your email deliverability and increase the ROI of your emailing campaigns.
Email deliverability refers to the frequency on which outgoing emails reach the primary folder of the recipient inbox, it can also be called "inbox placement". An email with low deliverability will have a high chance to land in the spam folder. And on the contrary, an email with high deliverability will surely make it to the inbox of its recipient.
Email deliverability is important as it affects the efficiency of your outreach strategy as a whole. Marketing campaigns (newsletters, content sharing, invite to webinars, etc.), cold outreach campaigns, or transactional emails (account verification, password reset, etc.) all need to be delivered in the inbox.
Emails that land in spam are not opened, and read. Period.
A low deliverability means a low open rate, low engagement and disastrous ROI. This is why you need to have the best email deliverability possible. And this is why you're here, reading this guide. And trust me, you're at the right place.
This is going to be a long article, based on more than 5 years of experience as an "Email Deliverability Expert" helping businesses to optimize the efficiency of their email campaigns. I'm going to share with you everything I know, compiled in a complete guide, including practical tips and tools that can help you to reach 100% deliverability. The Holy Grail for an email marketer.
Take a big cup of coffee and make yourself comfortable. Here we go!
I usually compare email deliverability with SEO (Search Engine Optimization).
It appears as a black box, implying many factors and evolving constantly. It seems that no one understands, or at least almost no one. This is due to the fact that email deliverability relies on complex algorithms called spam filters, managed by ISPs like Google or independent organisms. To understand how email deliverability works, you need to understand how spam filters are working. Just as you need to understand how Google search engine algorithms are working to understand SEO.
Spam filters are the algorithms that separate good emails, from bad ones. Placing the bad ones in spam filters and allowing access to the inbox to the good ones.
Most ESPs (Email Services Providers) are using several spam filters to sort the emails that are exchanged every day. And it's actually for good.
Spam filters protect us from abusive messages, scams, and inappropriate content that you don't want to find in your inbox, between two emails of your clients, colleagues, or friends.
From what we know, those filters are taking into accounts 3 main factors, to decide if an email is worth reaching the inbox of the recipient, or not.
The 3 key stones of email deliverability:
1) The reputation of the sender
2)The activity of the sending inbox
3) The content of the incoming email
We're going to deep dive into all of these factors, and analyze what they're made of.
But before anything, you need to keep in mind the objective of spam filters.
Find and terminate spammers.
This is the only thing that matters.
This is why you shouldn't spam, even if the content of your emails is relevant and even if it's not your intention.
Keep in mind that if you act as a spammer, you'll be considered as one.
This being said, we can deep dive into the first factor that impact email deliverability: the Sender Reputation.
Email deliverability is a matter of trust and reputation. Spam filters base their computations on external signs of trust of your inbox, domain, and IP to tell if your emails are spam or not.
Sender reputation refers to the trust "score" given by ISPs (Internet Service Providers) to domains and IPs that send emails. The higher it is, the higher will be the email deliverability. If it's low, ISPs will consider your domain/IP as suspicious and your emails will land in spam.
In this part, we're going to cover the main aspects of what define your sender reputation, and provide you with actionnable tips to help you improve it.
Here's a quick overview of the main things that you should monitor to have a high sender reputation:
But before anything, it's important that you know where you are, and identify what's wrong with your sender reputation.
Sender reputation can be difficult to understand, to evaluate. Having an idea of the state of your sender reputation and how it impacts your campaign's performances can be very valuable for you and your business.
You can use tools like Warmbox" Inbox Spam Checker", to your sender reputation and identify what's wrong with it (IP/Domain blacklists, Reverse DNS, SPF, DKIM, DMARC, etc.).
Such a tool will provide you with a deliverability score as well as a complete report, detailing what's wrong with your inbox.
Find out how to run an email deliverability test in this article.
Another interesting indicator to follow is your SpamAssassin score.
Apache SpamAssassin is a filtering program that uses a variety of spam-detection techniques and provides a spam score to every email: 5 being considered as spam.
SpamAssassin is used by most ISPs (Internet Service Providers) and individual domains as a spam filter. Meaning that a bad grade (4 or above) will impact severely your deliverability.
SpamAssassin developers keep the factors involved in the score calculation public.
These are some key elements checked by SpamAssassin:
However, SpamAssassin is a complex and dynamic program, including scores from other spam filters in its own score calculation.
The best way to improve your SpamAssassin Score is to follow clean emailing practices:
Now that you know how to identify the state of your sender reputation. Let's dive into the main items that are considered in the computation of your sender reputation.
Domain Blacklists are real-time databases that identify domains that are known to send spam or malicious content. They are defined by ISPs (Internet Service Providers) and specialized organisms and are used as references by email servers to determine if an incoming email can be trusted or not.
Email domains that are blacklisted are likely to land in spam, as they have a history of negative behaviors.
Your domain has risks to be added to a blacklist if:
Good sending practices and active authentication protocols (SPF, DKIM, and DMARC) can help you to reduce the risks to have your domain blacklisted.
Having your domain blacklisted will severely impact your deliverability, as most ISPs and ESPs (Email Service Providers) filter emails based on blacklist records. This is why you don't want to see your name on those.
Better safe than sorry. Clean emailing activity will ensure you to stay away form blacklists.
But in case this happens, you have two different options:
IP Blacklists are real-time databases that identify IP addresses that are known to send spam or malicious content. They are defined by ISPs (Internet Service Providers) and specialized organisms and are used as references by email servers to determine if an incoming email can be trusted or not.
Email IPs that are blacklisted are likely to land in spam, as they have a history of negative behaviors.
Your IP has risks to be added to a blacklist if:
Good sending practices and active authentication protocols (SPF, DKIM, and DMARC) can help you to reduce the risks to have your IP blacklisted.
Having your IP blacklisted will severely impact your deliverability, as most ISPs and ESPs (Email Service Providers) filter emails based on blacklist records. This is why you don't want to see your name on those.
Better safe than sorry. Clean emailing activity will ensure you to stay away form blacklists.
But in case this happens, you have two different options:
Not all domains are equals when it comes to email deliverability. One important factor taken into account by spam filters and Email Service Providers is the domain age.
It shows how long your domain has existed.
As a rule of thumb, the older is your domain, the better will be your deliverability.
Indeed, most ESP will consider that brand new domain are less trustworthy than aged ones.
Domain that has been created less than 3 months ago are more likely to have deliverability issues than older ones.
Yet, having an old and well established domain doesn't mean that your deliverability will be high.
It's only one factor among others. And it is important to always keep in mind that an healthy email activity is the best way to keep a high email deliverability.
Just as the domain age impacts your sender reputation, your domain name is of highest importance.
Thus, the TLD (Top Level Domain) of your domain name is the first aspect that you should cover. ".com" domains are the more trustworthy and on the opposite ".xyz" or ".biz" domains are considered as potentially spammy. And beyond the screening of the spam filters, the perception of the users also matters.
Would you trust more an email coming from "hello@company.guru" compared to "hello@company.com"?
But TLD is not the only component of your domain name. The rest of the domain is also very important. And to that matter, it's important to avoid some negative keywords, reported and known as potentially spammy.
Spam filters use keyword databases to spot suspicious domain and can lower their deliverability.
We wrote an article referencing 400 of the most common spam-trigger keyword to avoid, both in your emails and in your domain name to maximize your email deliverability.
You can access this article here.
SPF stands for Sender Policy Framework. It's an email authentication protocol that is published in the DNS records of a sending domain.
It gives a list of IPs, mail servers, and sending applications that are authorized to use this domain to send emails. As this list is public, the receiving server will be able to compare it with the IP or sending application of the incoming email.
It allows:
As for any email authentication protocols, having SPF properly set improves the deliverability of emails sent from this domain. Indeed, some email servers can block the access to email with no SPF record published.
In the same way, your domain has less chances to be blacklisted or have a bad SpamAssassin grade if it has a SPF record published.
In a nutshell, having SPF properly setup will make your emails more secure while increasing your deliverability.
I own the domain "mailook.ai" and I'm using Google Workspace to send emails, as well as Sendgrid for marketing purposes.
I will publish this SPF record in my DNS:
v=spf1 include:_spf.google.com include:sendgrid.net ~all
We can decompose this SPF record into 3 parts:
I. The version of the SPF protocol. It always stays the same.
v=spf1
II. The "list" of authorized applications and/or servers that can send emails with the domain "mailook.ai".
include:_spf.google.com include:sendgrid.net
III. The mechanism used. Basically, it indicate what will happen if an email using the domain "mailook.ai" is sent from an unauthorized sending application or IP.
~all
In that case, the email that doesn't pass the SPF test will be automatically sent in spam.
You need to publish a TXT record on your DNS to enable SPF authentication.
a. Name/Host/Alias: Enter @ or leave blank
b. Time to Live (TTL): Enter 3600 or leave the default
c. Value/Answer/Destination:
v=spf1 include:{{thirdparty.com}} ~all
{{thirdparty}} being the code snippet corresponding to the third party tools that you are using to send emails.
It can be Email Service Providers like Google Workspace, Outlook, but also Email marketing tools like Mailgun, Sendgrid, Sendinblue or Mailchimp.
Please refer to our code list to find the code snippet corresponding to the tools your using.
Example:
I own the domain "" and I'm using Google Workspace to send emails, as well as Sendgrid for marketing purposes.
I will publish this SPF record in my DNS:
v=spf1 include:_spf.google.com include:sendgrid.net ~all
DKIM stands for DomainKeys Identified Mail. It's an email authentication protocol, using digital signature attached to the header of email. In a nutshell, when DKIM is published in the DNS records of a domain, it will display a public key that the recipient email servers can match with the encrypted key in the signature.
It allows:
As for any email authentication protocols, having DKIM properly set improves the deliverability of emails sent from this domain. Indeed, some email servers can block the access to email with no DKIM record published.
In the same way, your domain has less chances to be blacklisted or have a bad SpamAssassin grade if it has a DKIM record published.
In a nutshell, having DKIM properly setup will make your emails more secure while increasing your deliverability.
You need to generate a pair of keys and to publish a TXT record on your DNS to enable DKIM authentication.
DMARC stands for Domain-based Message Authentication, Reporting & Conformance. It's an email authentication protocol, using Sender Policy Framework, (SPF) and DomainKeys identified Mail (DKIM) to prevent "domain spoofing" and other malicious activities.
It is published in the DNS records so that any receiving email server can authenticate incoming emails. It also details the sender policies on how unauthenticated emails should be treated by ESPs (Email Service Providers).
It allows:
DMARC is an extension of existing email authentication methods like DKIM and SPF. It makes sure that no unauthenticated users will be able to send email from your domain, protecting your brand and the trustworthiness of your domain.
Along with SPF and DKIM, DMARC has a positive impact on deliverability as major ISPs (Internet Service Providers) considered sender with DMARC set up as more trustworthy.
In the same way, your domain has less chances to be blacklisted or have a bad SpamAssassin grade if it has a DMARC record published.
In a nutshell, having DMARC properly setup will make your emails more secure while increasing your deliverability.
You need to publish a TXT record on your DNS to enable DMARC authentication.
a. Name/Host/Alias: _dmarc
b. Time to Live (TTL): Enter 3600 or leave the default
c. Value/Answer/Destination: Replace {email} with your email address.
v=DMARC1; p=none; rua=mailto:{email}; pct=100; sp=none
You can also generate a DMARC record using this tool: DMARC Record Generator
A Reverse DNS lookup is a query that will retrieve the domain associated with a given IP address. It's simply the opposite of the well-known DNS lookup which gives IP addresses associated with a domain.
Reverse DNS lookup is commonly used by email servers to track down malicious messages and spam, as it can identify and verify the domain of an incoming email.
Reverse DNS lookup is a key element of the security protocol used by email servers to protect recipients from malicious emails.
Not all sending servers are supporting Reverse DNS lookup, but those that don't are more likely to be blocked by receiving servers. This affects negatively your deliverability.
Yet, all major ESPs (Email Service Providers)'s servers do provide Reverse DNS.
You need to publish a PTR record on your server to enable Reverse DNS.
If you use a custom SMTP server to send your email, you should make contact with your provider that will provide you with the guidance to set Reverse DNS for your sending server.
Last but not least. The activity of your inbox is also very important.
In parallel with your sender reputation, mainly linked to your domain and IP and to your email content mainly linked to the email itself; inbox activity is the history of your email address and is used as an indicator by spam filters to determine your email deliverability.
Having an active inbox, with regular and healthy records is the best way for you to reach the maximum deliverability.
Here are the tips we can give your considering inbox activity:
Email warm-up is a process that consists of gradually increase the number of emails sent by an inbox and generate interactions with these emails. As a normal person that creates an email address would do, emails are sent, get opened, answered, marked as important, etc.
This process comes with an increase in the deliverability (i.e. the ability to land in the inbox instead of spam) of the email address. Thanks to the email warm-up, the ISPs (Email Service Providers) will identify you as a good sender and won't put your outgoing emails in spam.
All these actions can be performed by human beings, but can also be automatized. In that case, the warming inbox will interact with other inboxes to reproduce human behavior, with the objective to improve its deliverability.
Email warm-up is essential for anyone sending emails. Whether cold outreach campaigns, newsletters or other marketing emails, and even transactional messages.
At Warmbox.ai, we identified 3 types of user for whom email warm-up as been game changer:
Nothing could be easier. All you need to do is to create an account on Warmbox.ai and create a warm-up campaign for your inbox. All it takes is 2mn to set everything up and start improving your email deliverability.
Learn how to launch your first warm-up campaign with Warmbox int his article.
Don't hesitate to reach us on our live chat or by email at contact@warmbox.ai to get more details about email warm-up.
Having a perfect email activity means avoid suspicious gaps in your daily sending volume. It sounds basic but it's the core of the issue.
Would you find normal that an inbox that sent 10 emails on Monday send 400 on Tuesday? Spam-filters don't.
This is why it's crucial to keep your sending volume as steady as possible over time. This can be easily achieved thanks to email sending tools that enables you to set up your campaign's sending volumes on daily basis.
In the same spirit, if you plan to increase you sending volume, to do cold outreach for example, it's important to increase your daily sending volume progressively. To avoid huge gaps in your inbox activity and make sure your emails won't land in spam.
Email warm-up tools like Warmbox.ai are a good way to maintain this activity steady, as it will autmoatically send warm-up emails every day, at regular pace.
All ESPs allow you to send a limited amount of emails every day. Above this limit, additional emails won't be delivered. And worse, if you're playing too much with the limits, your ESPs can devaluate your overall email deliverability for excessive volumes.
The best is to be aware of the limits related to your ESP before starting a mass emailing strategy.
For example, the limit is set at 2.000 for a Google Workspace (ex Gsuite) Gmail account. The limit is set at 10.000 recipients every day for Outlook 365.
It can be also a good call to check what email service provider best fits your need in terms of sending limits.
Bouncing is one of the worst thing that can happen to your emails besides landing in spam.
In email marketing, a bounce correspond to an email that has not reached the recipient. At the difference with spam, that reached the recipient but not the inbox, a bounce won't be present at all in the recipient inbox.
A email will bounce if:
You should avoid bounce for two reasons.
An email that bounces won't reach it recipients at all. And as for spam, it won't generate value as it won't be read.
If it has a too high bounce rate, your inbox reputation can decrease, and affect your overall email deliverability.
The best way to reduce your bounce rate to the maximum is to keep your mail list as clean and fresh as possible. To avoid wrong email addresses you can verify them thanks to tools like TheChecker, DropContact, or NeverBounce.
It's also important to audit your mail list constantly to make sure that all the addresses that are inside are still valid and won't bounce. I also strongly recommend you to avoid using bought mail list or to be very careful when using this type of database, as they can be aged and inaccurate.
Besides the reputation of the sender, spam filters also take into consideration the content of the email itself. They use basic verification points to identify easily scams and spam attempts.
And in the same way, the final recipient of your message will also act as a spam filter, reporting your email as spam, if the content is not relevant enough, or appear as spam to him.
It's important to note that having too many people reporting your emails as spam can have a very negative impact on your deliverability as a whole, as it also impacts your sender reputation.
This is why, regarding your email content, you have to be careful about both about the analysis of spam filters (basically robots) but also to the perception of people that will receive your emails.
We're going to detail in this part the main aspect of your emails' content that should be taken into account to avoid both automatic spam filters and spam reports from recipients.
As a quick overview, those are the key elements you have to care about regarding your email content:
Spam filters use databases of negative keywords, associated with spammy or suspicious activity. For instance, an email starting with "WIN A FREE ACCESS TO OUR INCREDIBLE TOOL TO BECOME RICH" is very likely to be marked as spam by filters because of its semantic.
We built a curated list of 400 spam-trigger keywords that you should avoid using in your emails.
Find out the list in this article.
Make sure to avoid using those spam-trigger words in your emails (subject and body) is a good start to improve your content-based email deliverability.
The relevance of the content you send to your mail list is key to maximize both your deliverability, but also the ROI of your campaigns (being marketing or sales ones).
Indeed, if your recipients don't find your email interesting, and feel spammed, there are likely to report your email as spam, which can endanger your sender reputation.
In the same way, poorly personalized and irrelevant content won't bring a lot of answer/conversion/value.
Add the name of your recipient, its position, and maybe some other custom information is a good way to make your approach more human, and less spammy to the eyes of your recipients.
Most email sending tools enables you to add variable to your email campaigns in a simple way.
If you don't want to hyper personalize your email for every single recipient, you can also segment your mail list, based on the industry of the recipient, its level of development, its position, to have a more personalized approach that has fewer chances to be reported as spam by the recipient.
Email Service Provider can refuse to deliver your email if it contains a too heavy attachment.
In the same way, a mass sending with heavy attachment can trigger spam filters, as it's considered a suspicious/dangerous activity. Remember that spammers and scammers use emails to spread malicious files in attachments.
If you need to share a file with your recipient, we recommend that you use a Google Drive or Notion link instead of a PDF or other attachment.
A broken link or dead link is a hyperlink on a web page that no longer works, and redirects the users that click on it to a 404 error page.
A link can be broken for different reasons:
Broken links in emails are very deceptive for the recipients. As they will be redirected to an error page, they won't access the content you wanted to share with them.
Thus, emails having broken links inside are more likely to be reported as spam by recipients. Which can have a strong negative impact on your email deliverability.
Some spam filters are also tracking broken links and can directly place your email in spam if it contains such dead URLs.
The best practice is to always verify the links you send through emails, or to test your emails with tools like Warmbox "Email Spam Checker".
A short URL is a link that has been made shorter and still redirect to the required page. Usually, a user can generate short URL thanks to URL shortener tools (the most famous of them being Bitly).
Short URLs are useful as they allow click tracking and branding of the links shared.
URL shortener are great tools for marketers, but also for spammers and scammers.
Indeed, it allows them to hide the destination page of a link, and to spread malicious links.
This is why most spam filters are tracking short URLs inside emails and can put your message in junk if they detect a link that has been shortened.
Not only the use of a short URL will decrease your deliverability for given emails, but it will also impact your sender reputation and have a disastrous impact on your email deliverability as a whole.
It is common that people sending emails containing short URLs end up in domain or IP blacklists, and got their deliverability really hurt.
List-Unsubscribe is an optional email header that allows email recipients to opt-out of receiving emails without clicking through an unsubscribe link or marking a sender as spam or junk.
It is displayed next to the email sender information.
List-Unsubscribe Header is a user-friendly way to offer an opt-out for your recipient. Thus, they won't have to search for a unsubscribe link in your email body.
Offering such a feature in your emails is a good way to avoid that your recipients mark your email as spam to not hear from you again. Which is very damageable to your email deliverability.
To wrap everything that has been said, email deliverability mostly relies on the implementation of good emailing practices, combined with effective warm-up and proper technical setup.
Here are some tips and hints, to easily improve your email security, reduce the risk of being blacklisted, avoid being reported as spam by your recipient, or prevent spam filters from triggering. Having hygienic sending practices will improve your deliverability as a whole and help you to stay away from spam folders.
Here are the takeaways tips that can help you to reach 100% deliverability:
Don't hesitate to reach us on our live chat or by email at contact@warmbox.ai if you need custom deliverability consulting.
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