What is spam? How to prevent emails from landing in the spam folder?

What is spam? How to prevent emails from landing in the spam folder?

Although email marketing has been used for many years, the line that separates spam from what is not yet is interpreted in different ways (and often ignored).

In this article, we are going to see not a standard definition, but we will try to understand the term from a practical point of view.

And what's more important:

  • How to assess the potential chances of your newsletters ending up in the spam folder?
  • And how can you avoid it (within what is technically and legally possible)?

This is very important because, as you can imagine, the emails that land in the spam folder are much less likely to be opened.

And emails that are not opened are much less likely to generate sales.


First, what is spam?

Unsolicited email is the first known definition. Unwanted emails. Unwanted publicity, in short.

These are advertising emails with information that we didn't request and that someone (which we probably don't know) sent us, probably because they were trying to sell something.

Email providers try by all means to terminate or reduce the amount of spam their users receive.

Likely, you have already received one of these emails, right?

And it's also possible that you simply ignored it, or your email client sent it directly to the spam folder.

SPAM vs. phishing

We should not confuse spam, which is very annoying, with phishing, which is much more dangerous.

Spam is an unsolicited message sent with the intent to sell something.

On the other hand, phishing emails are sent to steal private information, such as passwords or credit card information.

The problem is that many times, as the sender is unknown, it is impossible to know the ultimate intent of an unsolicited message.

And that's why email providers (ESPs) are so careful to avoid unsolicited messages from reaching the inbox.

Why would anyone want to send spam?

As we mentioned before, most of the time, the intention of a spammer is to try to promote a product and sell it.

It is equivalent to unsolicited phone calls, or flyers sent by postal mail, that is, cold contact techniques, in which the client doesn't know the company that is contacting them.

The origins of spam are as old as email, and therefore, there was a struggle between the ESPs and companies that send spam from the beginning.

While ESPs try to limit the reach of spam messages, companies that send spam try to circumvent existing filters.

Unlike phishing, where an attempt is made to steal information, spam is just a sales strategy. Although, of course, these two terms are often confused.

How do ESPs and email clients know which emails are spam?

For example, how does Gmail know that a specific email sent to you is spam?

How can Gmail know which lists you're subscribed to? or if you requested information about a product or service?

The answer is that it is not possible, so to decide it, the destination servers (ESPs, etc.) use:

  • The email content > the subject line, text, words, links (and the destination of the links), images…
  • The sender of the email > what is the “lifetime” of the domain. If you have an old website, what information has been published on that site.
  • Historical information > this data is also associated with the sender. How many emails have been sent in the past? Of these emails, how many were flagged as spam by the people who received these messages? How many subscribers opened the newsletter?
  • Recipient information > if you have opened emails from a sender in the past, subsequent emails are less likely to be sent to the spam folder.

So we have, on the one hand, the content of the email itself and, on the other hand, a concept that we might call "sender reputation."

Therefore, this decision is very much based on what the sender in question has done in the past.

What to do if Hotmail, Gmail, Yahoo, or AOL consider a newsletter as SPAM?

Approximately 20% of legitimate emails never reach their destination. (Tom Sather, Return Path) Because they are not delivered, they end up in the spam folder, etc., affecting the results of our mass mailing campaigns.

This may seem like a very high volume at first, but fortunately we can influence these results. According to Tom Sather, only 17% of the emails that are not delivered or go to SPAM, of the total 20% that we mentioned before, are for reasons related to the content of the newsletter, the rest are rejected or considered as SPAM due to reasons related to the sender domain reputation.

The quickest way to try to influence the result is through the newsletter content, so before sending an email campaign, we have to check the SPAM score of the message. With Mailrelay that is a very simple process; you can check the SPAM score of your newsletter while you are still editing the message, in addition to following the other tips that we will see in this article.

This way, your campaign will be ready, with the content corrected, to avoid becoming one of the 17%. 17% of 20% might not sound like a lot, but correcting errors in email content is relatively easy.

How can you prevent your messages from being considered spam by Gmail?

Gmail is one of the most popular email services in the world, and it's important to ensure that your emails reach your subscribers' inbox and not the spam folder.

Here are some tips on how to prevent Gmail from considering your emails as spam:

Email authentication

Make sure you configure the SPF, DKIM and DMARC record correctly. This will help Gmail verify that you are the legitimate sender of the email.

Contact list

Make sure you only send emails to subscribers who have consented to receive them. Avoid buying mailing lists and prioritize the quality of your contacts rather than the quantity.

Relevant and quality content 

Create emails with quality, personalized content for your subscribers. Avoid excessive use of keywords commonly used by spammers or an overly promotional design.

Maintain a good sender reputation

Send emails regularly and avoid sudden spikes in the number you send. Maintain a good open and click rate and avoid having a high percentage of emails flagged as spam by users.

Give your subscribers an easy option to unsubscribe from your emails. This will help maintain a healthy contact list and reduce the risk of your emails being flagged as spam by users.

Check your spam score

Use tools such as SpamAssassin to assess the spam score of your emails before sending them. This will allow you to detect problems and optimize your emails to improve the delivery rate in Gmail.

What to do if your campaign is flagged as SPAM for reputational reasons?

Your domain reputation is something you should nurture every day from the first day you start working with email marketing. there are certain points that we must be very careful about, which we can summarize in:

  • Essential settings, such as SPF and DKIM. You should also avoid using hidden WHOIS.
  • Origin of the email list, you should work only with double OPT-IN or at least OPT-IN email lists. Subscribers should have voluntarily signed up for the mailing list. This process is vital as you must confirm that subscribers have agreed to receive your emails. If they want to receive your mass mailing campaigns, they will be less likely to flag your emails as spam.
  • Offer a clear option to unsubscribe from the list. You should use a direct unsubscribe link, with just one click. You need to understand that if it's easier to flag your campaign as SPAM than unsubscribe, they will flag it as SPAM. The problem is that it will affect your delivery rates in the future.
  • You should keep the list up to date, free from spamtraps and bounced accounts. Mailrelay helps you keep your list clean, but remember always to offer an easy opt-out option.
  • Use an adequate infrastructure. It's not the same thing to send an email from your computer, using Outlook and your IP, than using an email marketing service configured to maximize results like Mailrelay.
  • Finally, it is necessary to take into account the interaction of your subscribers with the newsletters, if they open the emails, if they click on the links, if they download the images, and if they respond. all these activities can be taken into account when assessing your reputation

Is it possible to mislead antispam systems?

Surely you already thought that it is possible to deceive the antispam filters because you have already received some spam emails.

Without going into technical considerations, I'm afraid that trying to mislead, the different antispam services will not be the easiest way out. 

At least not for most companies. If you have a business and want to use email to sell, the solution is not to trick the antispam systems.

Email marketing versus spam: permission-based email marketing

Basically, you should only send mass mailing campaigns to people who want to receive your emails and have authorized you to send them your newsletters.

There are several ways to get this request, but the most common situation is that visitors who are on your site and want more information will register to receive your emails.

Or customers who have agreed to receive messages expressly and clicked on the confirmation link you sent them to make sure they really want to be subscribed to your campaigns.

These contacts are much more unlikely to flag your mass mailing campaigns as spam. Although on some occasions, it may happen:

  •  If the unsubscribe link is not visible enough or the unsubscribe process is too complex.
  •  Or they last received newsletters from you a long time ago, and they don't remember your brand anymore.
  • In very specific cases, a customer who is very dissatisfied with a product or service could flag an email as spam just for revenge.

Now we can see the actions that could cause your emails to be considered as spam.

If your domain is new, give it some time and use it

Another important and easy-to-follow tip. If your domain is new, because you just registered it, it's better that you use it, sending emails, before sending mass mailing campaigns.

For the first few weeks, and even months, use it to send the company's internal emails. And then, you can progressively start to incorporate small email campaigns. It would be better to create an opt-in form to attract new contacts. You will also be sending confirmation emails, creating a natural process that will demonstrate to filters that your domain can be trusted, improving your delivery rates.

The main point that could cause your email to be considered as spam is if you send too many emails before your domain is ready. ESPs are suspicious of new domains that send large amounts of emails.

Add new contacts to your list regularly

It is normal for new contacts to have a very good relationship and demonstrate interest in the messages, mainly because they demonstrated that they were interested in your project recently. This means they will open your emails and will click on the links. The level of interaction will be higher for the first newsletters sent to a new contact. Therefore, you must keep incorporating new subscribers little by little and routinely on your email list to improve your open rates.

The easiest way to do this is to include a subscription form on your site, although this is also one option that brings fewer results. Popups or landing pages usually give better results because you will usually drive already-engaged leads to these pages.

It's something simple to do, but it requires constant work. A job that will not only result in increased sales opportunities but also in maintaining reasonable open rates.

Clean your mailing list if you suspect the problem is in your database.

Keeping your email list properly clean is a process that takes time, but it is something that can be done. We have already published articles on our blog about the topic, so it's something you should check out. Email spamtraps are fake contacts that ESPs sometimes use to detect spammers. these emails may look normal and act like it (they open your newsletters and even click on the links).

These emails can be placed on web pages, and if someone copies them and send campaigns to these addresses, everything will seem normal, but it is not.

Therefore, the recommendation is to use only the emails of your contacts, customers or emails you received from subscription forms (double opt-in).

The next step, review email content and spam score.

Once you start sending only to contacts with permission, can you be sure that all emails will go to the inbox?

Theory tells us that it should be so.

Is this true?

We send something they want to the people who requested it.

Even so, as we mentioned earlier, the content of emails is also very important.

How can we review the email content to prevent it from being considered as spam?

When you are working on your email design with Mailrelay, you can make use of the “spam report” tool.

This tool, based on the well-known spamassassin system, will tell you if there is a problem with the content of your email, the sender domain, the links, etc.

This tool is available for free:

When you click on the button, a screen like this one will be displayed:

If there were a problem or something to fix, you would see the message here.

Sometimes these messages are very easy to understand and resolve, and other times the warnings are more complex. In these cases, you could contact our customer support department.

Correcting all points will help your message to get a more positive score for many different spam filters.

In the next section, we'll look at some common error messages that can be generated by the spam report.

Quick tips to avoid landing in the SPAM folder

Avoid using attachments. This is one of the simplest things you can do to improve your delivery and open rates. Don't attach files to your mass mailing campaigns. especially those that can be considered as more suspicious, such as zip files. They are the most likely ones to be considered as spam.

1- Don't create a newsletter using a single image, nor a large image created with several small files, like a mosaic. Never include one or more images alone or with very little text.

2- Avoid including text inside the images. The texts should be included directly in the body of the message.

3- Avoid using URL shorteners. The use of shorteners can cause your emails to end up in the spam folder, even if everything else is correct. ESPs can't review the landing page because it has been shortened and therefore it raises suspicions.

4- Also, don't add the URLs directly in the text; create a link with text, for example, “click here” or something similar.

5- Avoid using adult-themed terms as much as possible, subject lines related to pharmaceutical products, financial or monetary terms, and in general any terms or phrases that might match campaigns created by spammers.

6- Don't use strong colors, fonts, or large formats and flashy styles.

7- The subject line shouldn't be written in capital letters, with exclamation points, end in a question, be just one word or be too long.

The most common and probable causes of an email landing in the spam folder

The most common causes of an email falling into the spam or junk folder:

Email content

The content of the email is one of the main factors that influence whether an email is classified as spam or not. Emails that contain suspicious words or phrases, such as "fast money", "unique opportunity" or "make money", are more likely to be identified as spam.

Unknown sender

If the recipient of the email doesn't recognize the sender, they are more likely to flag it as spam. It is important to ensure that emails are sent from a recognized email address and that the following items are clearly identified the name of the company or organization in the "From" field.

Bad domain reputation

If the sender's domain has a bad reputation on spam lists, emails sent from that address are more likely to be identified as spam. It is important to follow email marketing best practices and avoid sending unsolicited or unauthorized emails to prevent the sender's reputation from being negatively affected.

If an email doesn't include a link for recipients to unsubscribe from the mailing list, it is more likely to be identified as spam.

Poorly formatted email

If the email is not formatted correctly or contains errors in the HTML code, it is more likely to be identified as spam.

Common Spam Report Messages and Possible Solutions

Some of these messages may be displayed either because of the content of the email or the subject line of the message.

For example:


It indicates that the newsletter was created with just an image with a link to an external page. The newsletter must have content, not just an image with a link.

HTML has a low text-to-image ratio

It indicates that the newsletter has too many images and too little text. We should find a balance between the two.

HTML: images with 0-400 bytes of words

There is text written on at least one image, not the body of the email. There is nothing to do in some cases, but this should be limited as much as possible. It would be better to avoid this type of situation and use only illustrative images.

HTML: images with 1200-2600 bytes of words

Similar to above, but it detected more text included in the images.


The newsletter content contains a large percentage of text in capital letters, so it could be considered as spam.


Similar to the previous problem, but it detected even more capital letters in the content.

Message is 10% to 20% obfuscated in HTML

The HTML code appears to be obfuscated. It is likely that if the text has been copied from Microsoft Word or other similar editors, some tags from the editor itself were copied together, creating a suspicious text that spam filters think could include code, which would cause it to be considered as spam.

HTML with unbalanced body and tags

This message informs us that an error has occurred in the HTML code of the email. It is likely that some HTML tag has not been closed correctly. These types of errors will affect the display of the message.


It indicates that there is a WordPress URL in the newsletter that may have been compromised or hacked.


The email includes words like “credit” in the text, along with other related words, synonyms, etc.


This message is displayed if you are using Google's URL shortener.


It indicates that the subject line of the email is in capital letters.


Indicates that a word related to the term “ambien” in the body of the newsletter is affecting the score.

Obfuscated URL

URL obfuscation is a method used by criminals who plan phishing scams and hide a fake URL in a link with a text that looks different. For example:

Click here to update your account:


And when we click, we end up accessing something like this:

Review everything we've discussed to avoid landing into the spam folder

The good practices we discussed and possible solutions, as well as the error messages that are displayed in spamassassin will help to improve the delivery rates for your emails.

Of course, it is not a 100% guaranteed method because, in the end, everything will always depend on third parties, the ESPs; however, it is useful to know what you can do to improve your results.

Therefore, it is always recommended to stop and do some revisions before you send a newsletter, such as checking the campaign's spam report.

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