How to avoid email spam filters

If you're wondering how to avoid email going to the spam folder this article is for you. Become an email deliverability expert now.

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So you're sick and tired of hearing about your beautiful emails landing in spam? We're glad you checked in. If you’re responsible for your organization’s email campaigns, you’re probably constantly thinking about how to avoid spam filters, spam words, and always landing in the inbox.

It can be a stressful and tedious job, but once you take a step back you’ll see that there is a clear and easy way to stop email from going to the junk folder.

We get it - there’s nothing we hate more than seeing great email go to waste.

According to Return Path’s 2017 Deliverability Benchmark Report one in every six emails sent fail to land in the inbox.

Almost 25% of email marketers now cite deliverability as one of their main barriers to effective marketing - an increase of 5% in two years, and yet only 6% of marketers are focusing on solving this issue.

There’s no point spending time crafting great content if your messages are never seen.

In this article, we’ll help you start off on the right foot and land your email to its intended destination. We’ll first help you understand what email spam is, what tactics you can employ to avoid the spam folder, what words and phrases to avoid, and finally how to run spam filter tests.

What is email spam and junk email?

Spam, or junk email, refers to malicious or unrequested email sent by “spammers” who want something from you or want to do something to you (e.g. attack your computer with a virus).

Of course there’s a difference between spam and spam folders, which is the repository of all things that email clients and Internet Service Providers (ISPs) consider spam. Not everything in the spam folder is malicious, and in fact a lot of it isn’t. That’s why it’s important to adhere to best practices to avoid being marked as spam yourself.

Malicious content has been all but removed by ISPs, who have strengthened their filters in the past few years. In the early 2000s, you were probably still dealing with nonsense in your inbox and it probably made you hate your inbox a little bit. Today though it’s unlikely that a true spam email will ever make it through to the inbox.

That said, it’s easier than ever for consumers to mark emails as spam through one-click buttons on clients like Gmail which will then store future email from this sender in the junk folder.

Gmail’s new feature allows users to easily unsubscribe from emails they no longer want to receive

Email deliverability definitions

Spam filter definition

There are a lot of checks that happen when an email goes through the server. ISPs (such as Gmail, Yahoo, AOL) have put filters in place to protect spam or malicious email from landing in a recipients’ inbox.

One thing to keep in mind is that deliverability is different from delivery. The two sound very similar, but shouldn’t be confused.

Deliverability rate is calculated as how many emails are inboxed out of the total sent. Delivery rate actually includes all email accepted by the ISP, which includes email that lands in the spam folder.

You can have a 100% delivery rate, but if your deliverability rate is only 45%, you’re missing out on a huge opportunity to communicate with your customers.

Soft bounce definition

A soft bounce is when an email is sent back to the sender, but only for temporary delivery issues. This can be because the user’s inbox is full, the server is down or the message is too big for the recipient’s inbox. GIFs are a great way of increasing engagement in your emails, but make sure they’re not too big!

Hard bounce definition

A hard bounce is when an email is sent back to the sender because it couldn’t be delivered for permanent reasons. It could be that the email address was typed incorrectly, or a fake one was entered because the subscriber was more interested in receiving the perk or offer for signing up than receiving your email.

This is why it’s always helpful to set up double opt-in.

How to get emails delivered to the inbox instead of the spam folder

How to stop emails from going to the spam folder

There are several best practices and tips that you can follow to improve your reputation and deliverability. We created a free white paper that lists 34 factors that can impact your deliverability, but also highlighted the top tips to follow and adopt here:

Sender addresses:

✗ Avoid using free web-based email addresses (Gmail, Hotmail, etc) for your sender addresses.

✓ Use a custom domain email address that is linked to your website. You will then be able to set up DKIM & SPF, which will allow for email authentication by the recipient servers.

✓ Ensure your website is active and running. Sending email from an address that is linked to an inactive or blank website will make ISPs suspicious.

Contact lists:

✗ Do not purchase, borrow or copy any third party contact lists. Not only do these types of lists typically contain many spam traps and poor quality email addresses, it is against our Acceptable Use Policy. Note: A spam trap is an email address that is not used for communication and it should never receive emails; if it does receive email, then it is automatically considered to be spam

✓ Develop good quality contact lists by collecting email addresses via an opt-in from your website. A double opt-in process is recommended to eliminate mis-typed or fake email addresses.

✓ Regularly update and clean your contact lists. Monitor your mailing results, and remove older non-engaging or blocked email addresses. Focus on the people who are most interested in your newsletters.

Email content:

✗ Do not use ALL CAPITALIZED WORDS in your subject line or body.

✗ Avoid using spammy type words (‘Free’, ‘Sale’, ‘Cash’, ‘Limited Time Offer’, etc). (more on words to avoid below)

✓ Keep your subject line between 35 to 50 characters long. The longer your subject line, the more likely it will be flagged as spam.

✓ Send content that your subscribers have signed up for and are expecting. If you send non-relevant content, your subscribers may mark you as a spammer. And the more people that open your newsletters, the better your reputation

✓ Send your newsletters consistently.

Want to know more about deliverability best practices? Download our guide now!


Landing in the inbox: Email marketing deliverability basics

Don’t let great emails go to waste. Discover everything you need to know to land in the inbox.

Advice on how to send bulk email without spamming

Sending bulk email that consistently lands in the inbox unfortunately can be a pretty frustrating process, especially if you’re not staying on top of your lists, campaigns, and sending processes.

That’s what Mailjet is here for, to not only optimize our platform for world class deliverability, but also to arm our customers with the latest tips and best practices to ensure deliverability.

Monitor your contact lists

We’ll begin where we often begin when it comes to email: your contact lists. The first place we look when a customer is having deliverability issues is their contact lists to determine (1) where these contacts came from, (2) if and how they are engaging with the content, and (3) whether the lists are being cleaned frequently.

If it hasn’t been stated enough - avoid buying lists or scraping the web for all costs. Not only will your deliverability suffer by reducing the reach of your emails to legitimate audiences but in a world increasingly concerned about data privacy, and in fact governments that are cracking down on this heavily, the only best practice here is to build your list organically.

Next, pay attention to how your users are engaging with your content. What are their open rates, bounce rates, and blocks? Without consistent oversight, it’s easy to let these numbers drift upwards and consequently see your deliverability drift downwards.

While some users who no longer want or need to receive your messages will unsubscribe, more often than not your subscribers will start ignoring your email, maybe even marking them as spam, or the inbox provider like Gmail and Outlook will start to filter out your messages on their users’ behalf.

As a result, the onus is on you to clean your lists and keep those open rates and deliverability rates moving up and to the right.

To do this, you can use Mailjet’s Segmentation feature to identify those users who haven’t opened your emails in a few months, or those who are marking it as spam.

Brands that keep their lists clean can actually see an increase in not only open rates but also total opens as a result of better deliverability.

Use email authentication with SPF, DKIM, and DMARC

Trust and permission is at the center of a successful email marketing strategy which makes it one of the most powerful marketing channels.

As a result, to avoid the spam folder, you need to prove to inbox providers like Gmail, Yahoo, and Outlook - the gatekeepers to your audience - that you are, in fact….you. Think of inbox providers as bouncers at a bar, they only care about two things: (1) do you have identification, and (2) are you worthy of connecting with the crowd inside?

In order to get passed the bouncer, you don’t need to slip a fake ID and a $20 bill, it’s a little more complex than that. Here are a few things to consider to authenticate your email, prove who you are, and get past the spam filters:

  • Sender Policy Framework (SPF) – is an email validation protocol designed to detect and block email spoofing by providing a mechanism to allow receiving mail exchangers to verify that incoming mail from a domain comes from an IP Address authorized by that domain's administrators.

  • Domain Keys Identified Mail (DKIM) – an email authentication method designed to detect email spoofing. It is a way to sign and verify email messages at the message transfer agent (MTA) level using public and private keys. The public keys are published in DNS TXT records. DKIM authenticates the source and its contents.

  • Domain-Based Message Authentication Reporting and Conformance (DMARC) – an email-validation system designed to detect and prevent email spoofing. It is intended to combat certain techniques often used in phishing and email spam, such as emails with forged sender addresses that appear to originate from legitimate organizations.

To set up your SPF & DKIM records, you will need to copy the SPF & DKIM values from your Mailjet account to your DNS records for the domain you want to authenticate.

You can follow our step-by-step guide to set up your SPF and DKIM records.

How spam words are triggering the spam filter

Over the past few years, we’ve seen ISPs become a bit smarter and have started to move away from more traditional spam alarms.

Today, ISPs learn from how we interact with the messages that arrive in our inbox, which helps them determine whether emails should go to the Spam folder or land safely in our inbox.

So what does this means for words you should (and should not) be including in subject lines, then? Well, words tend to be misleading, thus resulting in higher-than-normal user complaint rates. These complaints, along with poor interaction from recipients have a negative effect on the sender reputation and, ultimately, impact the deliverability of future messages.

Just imagine how many times you have received a subject line that includes the word “Free”. How many times has there actually been something that’s truly free in the email? Probably very few, which explains why now, when you read the word “Free” in your inbox, you generally just roll your eyes at a not-so-subtle attempt to get you to open a deceiving email.

And if people do open the email and then find that there’s actually nothing really free there, senders can expect a high rate of user complaints like spam reports and unsubscribes that will impact their future inbox placement.

If you are looking to avoid those words that will trigger spam filters, we’ve got you covered. Below is some common spam lingo to help protect you from using them yourself and being mistaken for a spammer, or worse, a phisher.


The word “invoice” is a phisher’s favorite – if you see this word in a subject line, there’s a chance they’re trying to bait you in. Make sure to check the sender address to verify the email’s validity. is not the same as Scammers try to profit out of our carelessness.

PayPal, Visa/MasterCard, or any bank name

Again a case where a legitimate name can be used for phishing.

Scammers often try to impersonate financial institutions by sending emails with the same color scheme and layout, redirecting to a mirrored site made to look almost exactly like the one it is spoofing.

As a consumer, follow the same steps above, verifying the sender address and domain name. As a marketer, use authentication tools DKIM and SPF to prevent spoofers from hurting your reputation.

Lottery, Free Gift, Prize

This is one you always see in your spam folder. Hundreds of thousands of emails are sent to people with a subject line claiming that they’ve just won a big prize or that they’ve been selected for a sweepstakes you’ve never entered before. You have to be very gullible to fall for that one, yet scammers still send these by the millions since they are quick and easy to send. If it’s too good to be true, then it is. When you craft your emails, don’t give your customers a chance to ask themselves this question and certainly don’t let the ISP ask this question.

Urgent, Desperate, Please Help

Variations of this “damsel in distress” scheme have made appearances over the years, where phishers pretend to be an affluent person from a far away country, who, being chased by wrongdoers, is forced to flee to a safe haven. For some reason they have chosen you as the sole trustee of all their money and they promise great rewards for helping them open an account with a specific bank so that they can transfer their funds. These spammers are the butt of many jokes, avoid these words to avoid being on the wrong end of the joke.

Casino, Free Spins, Deposit Bonus

Gambling spammers often send out campaigns that promise high returns, free entry or double deposits. If it’s not a website you recognize, then straight to the spam folder it goes.

Here are some examples of specific words you want to be cautious of using:

Words to avoid when sending your email 

How to prevent email from going to spam: Use spam checkers or spam filters testing

What is a spam checker or a spam filter test?

Even if you follow all of the above best practices, inevitably you may have missed something, or even more likely is that there might be something going on that you could have never caught with the naked eye.

That’s why it’s so important to run spam tests to check the potential of your email being delivered to both the ISP and the ultimate inbox.

Unlike the naked eye, or even your picky colleagues’ eye, a spam test reviews your email to determine whether different spam filters will flag it and keep it out of inboxes. The test looks at everything from the content of your email, subject lines, where you are sending it from, and your domain's reputation. To use our bouncer analogy from before, it’s like showing up hours before the evening gets started to have your bouncer pre-approve you for access. It might not always work but it certainly gives you some assurances you didn’t have before.

How do I run a spam test?

If you’re using an Email Service Provider like Mailjet to send emails, then your best bet is something called a seed list. A seed list is a list of internal emails you can send a test email to, such as co-workers, family members or friends.

Ideally, you’ll want the email address to cover a range of email clients and devices, so you can check if it makes it through the different email spam filters.

Using Mailjet, before you send your email to the masses, you can send a test email that not only tests for spam filters (like Gmail spam filters) but also is a great way to test for email responsiveness in different clients. To best use seed lists though, there are many services designed just for this purpose such as Litmus, Email on Acid, and many more.

Each of these tools will provide you with a seed list of email addresses that you can cut and paste into your test email and send out to identify any issues with landing in the inbox. Spam testers will test for the following flags:

  • Email Server Reputation

  • Sender Email Address

  • Sender IP address

  • Email Server Configuration

  • Email Content and Subject Line

  • IP Blacklisting

As an example, Email on Acid is an email optimization tool which includes a spam filter test. Using any spam tester, alongside Mailjet, you can test your emails using the following easy step-by-step process:

  • Create your email and of course first check for any red flags in the content, subject line, and contact list

  • Once you think you’re ready to send, click on Send a Test Email

  • In your Spam Testing tool, select Start a New Spam Test or Start Spam Test

  • Copy and Paste all of the seed contacts that Email on Acid generates into Mailjet

  • Send your test email

  • Go back to your Spam Test Tool to identify any spam warnings and understand how you can continue to optimize your campaign to ensure maximum deliverability.

These services will send your emails through all the major spam filters before sending to make sure that they pass the first test.

Then it will check your sender reputation by looking at your IP addresses and any domain names used in your email.

There are many known blacklists and if your reputation is at all compromised or flagged, you’ll get a notification before sending. Next it will verify that your email authentication, such as DKIM, DomainKeys, SenderID, and Sender Policy Framework, is set up properly.

Finally, some services even provide you with a spam score, so you can compare your campaigns against past campaigns and your colleagues' campaigns. The root of all happiness? Quantifiable competition.

Want to know more about how to get your emails delivered to the inbox? Check out our Email Growth Playbook, a database of 60+ tactics to help you increase the performance of your email campaigns.


Email Growth Playbook

Explore tactics to increase your email ROI

Avoid the spam folder with Mailjet

Mailjet is constantly looking for ways to optimize and improve the deliverability of our customers’ emails. We manage the reputation of each sender and provide authentication tools (SPF, DKIM, etc.) to help implement all of the above best practices. We also optimize sending frequencies (i.e. throttling) and HTML code.

Track deliverability rates and contact management in Mailjet’s Stats page

Ultimately, Mailjet is designed to simplify the whole process of sending emails and ensuring deliverability, so that our customers can focus on sending great newsletters, transactional emails, or whatever other content you want your audience to engage with. But what does Mailjet actually do? Let's dig into 5 key points:

1. Management and monitoring of the reputation to avoid the spam folder

Mailjet allows you to watch and protect the global reputation you have as a sender. This is defined by the reputation of the URLs, the domains and the IP addresses that are used. The content of the messages can impact each of these elements.

A lot of indicators are provided on the Mailjet Dashboard, and you will have access to the reputation of your IPs and to the scoring SpamAssassin, which validates the major formats and filters.

We have also partnered with third party tools to help manage deliverability and reputation stats - the more you know about what practices aren’t working, the more you can start to solve them.

2. Avoid the spam folder - Access to authentication tools: SPF, DKIM & Domain Keys

Authentication systems have a set of standards for most of the ISPs. These protocols guarantee and protect the identity of the senders as well as fight against phishing.

Therefore, it is often necessary to publish these certificates. If this is not done, the ISPs can consider the non-authenticated emails as suspicious and place them in the spam folder.

Mailjet implements and optimizes all major email authentication protocols that senders need including DKIM (DomainKey Identified Mail), SPF (Sender Protection Framework) and DomainKeys by default. This can also be personalized for free. If you require assistance in this process, please contact our support team.

3. Optimization of the sending pace (i.e. throttling)

ISPs use ‘throttling’ mechanisms to control the volume of data traveling over their networks. Some impose temporary or permanent volume restrictions.

The threshold is based on the number of connections between the sending server and the receiving server, the number of messages per connection, and the volume of messages over time.

If you attempt to open too many SMTP connections at the same time or send too many email messages within a short time, you are very likely to get errors such as:

  • ‘timeout’,

  • ‘server has exceeded the rate limit allowed’, or

  • ‘too many connections from your IP’.

If this happens, the reputation of the IP addresses used can have a major effect on these limits. In order to guarantee optimization of the reputation, Mailjet will slow down and adapt the sending pace when needed.

By respecting these imposed variations, the messages are accepted, they do not bounce and get to the inboxes of the intended recipients.

4. Optimization of the HTML structure of the email

Email building platform by Mailjet called Email Editor, gives anyone the ability to get 100% optimized code from the header to the footer of the email. All HTML elements abide by standardized rules and guarantee improved deliverability.

Note that when using this feature, no technical knowledge is required. Always remember that an incorrectly coded email can trigger some spam filters.

5. Statistics, Feedback Loops and the Relationship with the ISPs

Mailjet maintains highly accurate, real-time data to track every piece of feedback from audiences - including those who mark messages as spam, and emails that are blocked or bounce. Every complaint is traced and taken into account.

This is best illustrated when someone clicks on the button “report as spam” on any inbox client. Mailjet gets this information and stops sending to this email address.

The bounces and unsubscriptions are also automatically managed. This helps maintain high quality lists. Anyone who is persistent in sending undesired emails could get blacklisted at any moment.

Even if the complaints come only from 1% of the recipients, this would be enough to get blocked by the major ISPs. In general, Mailjet works closely with ISPs and respects all the latest standards and recommendations. Our Acceptable Use Policy helps set the standard for all senders.

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