Avoiding the spam folder is key to any email marketing strategy: if your newsletters don't reach the inbox, they don't get read. In this guide you'll learn what spam is, how spam filters work, the role of SPF, DKIM, DMARC and sender reputation, and how to use the spam score to improve your email deliverability.
Even though email marketing has been around for many years, the line between what is spam and what isn't is still interpreted in different ways (and often ignored).
In this article we won't look at a standard definition. Instead, we'll try to understand the term from a practical point of view.
And, most importantly:
- How can you assess the chances of your newsletters ending up in the spam folder?
- And how can you prevent it (within what is technically and legally possible)?
This matters because, as you can imagine, emails that land in the spam folder are much less likely to be opened.
And emails that aren't opened are much less likely to generate sales.
TABLE OF CONTENTS
- First of all, what is spam?
- How do spam filters decide which emails are spam?
- Email deliverability: what to do if Gmail, Outlook or Yahoo treat a newsletter as spam
- How to stop Gmail, Outlook or Yahoo from treating your messages as spam
- Sender reputation: what to do if your newsletter is flagged as spam
- Is it possible to trick spam filters?
- If your domain is new, give it time and use it
- Add new contacts to your list regularly
- Review the email content and the spam score
- The most common reasons an email lands in the spam folder
- Common spam report messages and possible solutions
- Summary: how to avoid the spam folder and improve deliverability
First of all, what is spam?
Unsolicited email is the first known definition. Unwanted emails. Unwanted advertising, in short.
They are advertising emails with information we haven't asked for, sent to us by someone (whom we probably don't know), most likely because they were trying to sell us something.
Email providers do everything they can to remove or reduce the amount of spam their users receive.
You've probably received some of these emails already, right?
And you may have simply ignored them, or your email client may have sent them straight to the spam folder.
Spam vs. phishing
We shouldn't confuse spam, which is very annoying, with phishing, which is much more dangerous.
Spam is an unsolicited message sent with the intention of selling something.
Phishing emails, on the other hand, are sent to steal private information, such as passwords or credit card details.
The problem is that, often, because the sender is unknown, it's impossible to tell what the real intention of an unsolicited message is.
That's why email providers take such care to keep unsolicited messages out of the inbox.
Why would anyone send spam?
As we said, most of the time the goal of someone sending spam is to promote a product and sell it.
It's the equivalent of unsolicited phone calls or flyers sent by post, that is, cold outreach techniques, where the customer doesn't know the company contacting them.
Spam is as old as email itself, so from the very beginning there has been a battle between email providers and the companies that send spam.
While providers try to limit the reach of spam messages, the companies sending it try to get around the existing filters.
Unlike phishing, which tries to steal information, spam is just a sales strategy. Although, of course, the two terms are often confused.
How do spam filters decide 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've subscribed to? Or whether you've requested information about a product or service?
The answer is that it can't, so to decide, email providers (Gmail, Outlook, Yahoo, etc.) use:
- The email content > the subject line, the text, the words, the links (and where they lead), the images…
- The email sender > how "old" the domain is, whether it is correctly authenticated (SPF, DKIM and DMARC) and, if you have an older website, what information has been published on it.
- Historical information > this data is also tied to the sender. How many emails have been sent in the past? Of those emails, how many were marked as spam by the people who received them? How many bounced? How many subscribers engaged with the newsletter?
- Recipient information > if you've engaged with a sender's emails in the past (opening them, replying, or moving them from the spam folder to the inbox), the next 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, a concept we could call "sender reputation".
This decision is therefore largely based on what that sender has done in the past and how its subscribers react to its emails.
Email deliverability: what to do if Gmail, Outlook (Hotmail) or Yahoo (including AOL) treat a newsletter as spam
According to Validity's 2026 email deliverability report (with 2025 data), about 1 in 8 legitimate marketing emails doesn't reach the inbox: the global average inbox placement rate was 87.2%. Of the rest, some end up in the spam folder and some are blocked or rejected by the provider, which affects the results of our bulk email campaigns.
In Europe the average is better (91.1%), but there are big differences between providers:
| Provider | Inbox | Spam | Blocked or missing |
|---|---|---|---|
| Gmail | 89.8% | 6.4% | 3.8% |
| Yahoo | 87.3% | 6.4% | 6.3% |
| Apple | 82.0% | 10.8% | 7.2% |
| Microsoft (Outlook, Hotmail) | 77.4% | 15.1% | 7.5% |
Fortunately, we can influence these results. Today, providers decide mainly based on sender reputation: domain authentication (SPF, DKIM and DMARC), the spam complaint rate, bounces and how subscribers engage with your emails. According to Validity, the improvement in 2025 was largely because senders started meeting the mandatory requirements of Gmail, Yahoo and Microsoft, which we'll look at next.
The newsletter content also matters, so it's worth reviewing it before sending a campaign. With Mailrelay this is very easy: you can check your newsletter's spam report while you're editing it. But a good content score alone doesn't guarantee reaching the inbox, so we recommend you also follow the rest of the tips in this article.
How to stop Gmail, Outlook or Yahoo from treating your messages as spam
Gmail, Outlook (Microsoft) and Yahoo account for most email users, and it's important to make sure your emails reach your subscribers' inbox and not the spam folder.
Here are some tips to achieve this:
Mandatory requirements for bulk senders
Since February 2024, Gmail and Yahoo have required minimum standards from anyone sending large volumes of email. Gmail considers a bulk sender to be anyone who sends around 5,000 or more emails a day to Gmail accounts, and keeps that classification even if volume later drops; Yahoo applies similar criteria. If the requirements aren't met, emails may go to spam or be rejected:
- Authentication: SPF and DKIM correctly set up and a published DMARC record (at least with the p=none policy).
- One-click unsubscribe: marketing emails must let people unsubscribe with a single click, and the request must be processed within two days.
- Few complaints: the rate of emails marked as spam by users must stay below 0.3%, and ideally not exceed 0.1%.
Since May 2025, Microsoft (Outlook, Hotmail and Live) has required SPF, DKIM and DMARC from anyone sending more than 5,000 emails a day to its accounts, and recommends the rest of the best practices, such as easy unsubscribing and keeping the list clean. Even if you send fewer than 5,000 emails a day, meeting these requirements is the best way to protect your deliverability.
Email authentication: SPF, DKIM and DMARC
Make sure your SPF, DKIM and DMARC records are set up correctly. This helps providers verify that you are the legitimate sender of the email. For DMARC, you can start with the p=none policy and, once you've confirmed that all your legitimate emails authenticate correctly, move to p=quarantine or p=reject.
Contact list
Make sure you only send emails to subscribers who have given their consent to receive them. Avoid buying mailing lists and put the quality of your contacts before quantity.
Relevant, high-quality content
Create emails with high-quality, personalized content for your subscribers. Avoid overusing keywords common among spammers or an overly promotional design.
Keep a good sender reputation
Send emails regularly and avoid sudden spikes in sending volume. Look after your subscribers' engagement (clicks, replies, visits to your website) and avoid a high percentage of emails being marked as spam by users. Keep in mind that the open rate is no longer a reliable indicator: some email clients, such as Apple Mail, load emails automatically and count them as opened even if the user hasn't read them.
Include clear unsubscribe links
Give your subscribers an easy way to unsubscribe from your emails: a visible link in the email body and, in addition, one-click unsubscribe in the message header, which is what lets Gmail or Yahoo show an unsubscribe button next to the sender's name. This helps keep your contact list healthy and reduces the risk of your emails being marked as spam by users.
Use an address people can reply to
Avoid "no-reply" addresses. Microsoft asks that the sender or reply-to address be valid and able to receive replies, and replies from your subscribers are a positive sign of interest.
Monitor your reputation with the providers' tools
The providers themselves offer free tools to check your reputation and complaint rate: Google Postmaster Tools (Gmail), Yahoo Sender Hub and Microsoft SNDS (Outlook).
Check your emails' spam score
Use tools such as SpamAssassin, which Mailrelay's spam report is based on, to review your emails before sending them. It will help you detect content, link or formatting problems. Keep in mind that Gmail, Outlook and Yahoo use their own filters, so a good score doesn't guarantee reaching the inbox: it's one more check that complements everything above.
Sender reputation: what to do if your newsletter is flagged as spam
Your domain's reputation is something you should look after every day, from the very first day you start doing email marketing. There are certain points we need to pay close attention to, which we can sum up as:
- Essential settings: SPF, DKIM and DMARC. They are mandatory for bulk sending to Gmail, Yahoo and Outlook.
- Source of the mailing list: you should only work with double opt-in lists or, at least, opt-in lists. Subscribers must have signed up to the mailing list voluntarily. This process is essential, since you need to confirm that subscribers have agreed to receive your emails. If they want to receive your bulk campaigns, they'll be less likely to mark your emails as spam.
- Offer a clear way to unsubscribe from the list. You should use a direct, one-click unsubscribe link, and also offer one-click unsubscribe in the email header. Keep in mind that if it's easier to mark your newsletter as spam than to unsubscribe, people will mark it as spam. The problem is that this will affect your deliverability in the future.
- Monitor your complaint rate: keep it below 0.1% and never above 0.3%. You can check it in Google Postmaster Tools, Yahoo Sender Hub or Microsoft SNDS.
- Keep your list up to date and free of spam traps and bounced accounts. Mailrelay helps you keep your list clean, but remember to always offer an easy way to unsubscribe.
- Use suitable infrastructure. Sending an email from your computer, using Outlook and your own IP, is not the same as using an email marketing service set up to maximize results, such as Mailrelay.
- Finally, you need to take into account how your subscribers engage with your newsletters: whether they click on the links, reply, move them from the spam folder to the inbox or mark them as spam. All these activities can be taken into account when your reputation is assessed.
Is it possible to trick spam filters?
You've probably already thought it's possible to trick spam filters, because you've received spam emails yourself.
Without going into technical details, I'm afraid that trying to trick the different anti-spam services won't be the easiest path.
At least not for most companies. If you have a business and want to use email to sell, the solution isn't to trick anti-spam systems.
Email marketing and spam: permission-based email marketing
Basically, you should only send bulk 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 permission, but the most common is for visitors to your website who want more information to sign up to receive your emails.
Or customers who have expressly agreed to receive messages and have clicked the confirmation link you sent them to make sure they really want to subscribe to your newsletters.
These contacts are much less likely to mark your bulk campaigns as spam. Although it can sometimes happen:
- If the unsubscribe link isn't visible enough or the unsubscribe process is too complicated.
- Or if they haven't received newsletters from you for a long time and no longer remember your brand.
- In very specific cases, a customer who is very unhappy with a product or service might mark an email as spam just out of spite.
Now let's look at the actions that could cause your emails to be treated as spam.
If your domain is new, give it time and use it
Another important and easy tip. If your domain is new, because you've just registered it, it's best to use it by sending emails before sending bulk campaigns.
For the first few weeks, or even months, use it to send the company's internal emails. Then you can gradually start adding small email campaigns. It's best to create an opt-in form to attract new contacts. You'll also be sending confirmation emails, creating a natural process that shows filters your domain can be trusted, improving your deliverability.
One of the main reasons your email might be treated as spam is sending too many emails before your domain is ready. Email providers distrust new domains that send large amounts of email.
Add new contacts to your list regularly
It's normal for new contacts to have a very good relationship with you and to show interest in your messages, especially because they recently showed interest in your project. This means they'll open your emails and click on the links. Engagement will be higher in the first newsletters sent to a new contact. That's why you should keep adding new subscribers to your mailing list gradually and regularly to maintain good engagement.
The easiest way to do this is to include a subscription form on your website, although this is also one of the options that gets the fewest results. Popups or landing pages usually work better, because you normally direct leads who are already interested to those pages.
It's simple to do, but it requires constant work. Work that will translate not only into more sales opportunities, but also into keeping your subscribers well engaged.
Clean your mailing list if you suspect the problem is your database
Keeping your mailing list clean is a process that takes time, but it can be done. We've published articles on this topic on our blog, so it's worth checking them out. Spam traps are fake contacts that email providers sometimes use to detect spammers. These emails can look normal and behave as such (they open your newsletters and even click on links).
These email addresses can be placed on web pages, and if someone copies them and sends campaigns to those addresses, everything will look normal, but it isn't.
Also keep in mind that providers close inactive accounts (Yahoo after 12 months and Gmail after 24), so sending to old lists increases bounces.
That's why we recommend only using the emails of your contacts, customers or the emails you've received through subscription forms (double opt-in), and regularly removing bounces and contacts who haven't engaged for a long time.
Review the email content and the spam score
Once you start sending only to contacts who have given permission, can you be sure that all your emails will reach the inbox?
In theory, they should.
Is that true?
We send something people want to the people who asked for it.
Even so, as we said earlier, the content of the emails is also important.
How can we review the email content to keep it from being treated as spam?
When you're working on your email design in Mailrelay, you can use the "spam report" tool.
This tool, based on the well-known SpamAssassin system, will tell you if there are any problems with your email content, the sender domain, the links, etc.
This tool is available free of charge:
When you click the button, you'll see a screen like this:
If there were any problems or anything to fix, you would see the message here.
Sometimes these messages are very easy to understand and fix, and other times the warnings are more complex. In these cases, you can contact our customer support department.
Fixing all the points will help your message get a more positive score in many different spam filters. Remember, though, that Gmail, Outlook and Yahoo use their own filters and give a lot of weight to sender reputation, so a good score alone doesn't guarantee reaching the inbox.
In the next section, we'll look at some common error messages the spam report can show.
Quick tips to avoid the spam folder
Avoid attachments. It's one of the simplest things you can do to improve your deliverability. Don't attach files to your bulk campaigns, especially those that may be considered more suspicious, such as zip files. These are the most likely to be treated as spam.
- Don't create a newsletter using a single image, or one large image made up of several small files, like a mosaic. Never include one or more images on their own or with very little text.
- Avoid putting text inside images. Text should be placed directly in the body of the message.
- Avoid URL shorteners. Using shorteners can send your emails to the spam folder, even if everything else is correct. Providers can't check the destination page because it's shortened, and that raises suspicion.
- Also, don't paste URLs directly into the text; create a link with text that describes where it leads, for example, "See the autumn catalog". Avoid generic text such as "click here", which is less accessible and gives the subscriber less information.
- As far as possible, avoid adult-themed terms, subjects related to pharmaceutical products, financial or money-related terms and, in general, any term or phrase that could match campaigns created by spammers.
- Don't use bright colors, large fonts or formats, or flashy styles.
- The subject line shouldn't be written in capital letters, overuse exclamation marks or be too long. Above all, it should reflect the real content of the email: misleading subject lines generate complaints and, in some places, such as several US states, can lead to penalties.
The most common reasons an email lands in the spam folder
The most common reasons an email ends up in the spam or junk folder:
Poor domain reputation
If the sender's domain has a poor reputation (many complaints, bounces or sends to spam traps) or appears on blocklists, emails sent from that address are more likely to be identified as spam. It's important to follow email marketing best practices and avoid sending unsolicited or unauthorized emails so that the sender's reputation isn't harmed.
Missing authentication
If SPF, DKIM and DMARC aren't set up correctly for the domain, providers can't verify that the sender is legitimate. For bulk sending to Gmail, Yahoo and Outlook, this can cause emails to be rejected outright.
Unknown sender
If the recipient doesn't recognize the sender, they're more likely to mark the email as spam. It's important to make sure emails are sent from a recognized email address and that the company or organization name is clearly shown in the "From" field.
Email content
The email content also influences whether an email is classified as spam or not. Emails containing suspicious words or phrases, such as "fast money", "once-in-a-lifetime opportunity" or "earn money", are more likely to be identified as spam.
Missing unsubscribe links
If an email doesn't include a link for recipients to unsubscribe from the mailing list, it's more likely to be identified as spam. For bulk sending, one-click unsubscribe is also mandatory.
Poorly formatted email
If the email isn't formatted correctly or contains errors in the HTML code, it's more likely to be identified as spam.
Common spam report messages and possible solutions
Some of these messages can appear because of the email content or because of the subject line.
For example:
HTML_SHORT_LINK_IMG_1
It means the newsletter was created with just one image linking to an external page. The newsletter must have content, not just an image with a link.
HTML has a low ratio of text to image area
It means the newsletter has too many images and very little text. We need to find a balance between the two.
HTML: images with 0-400 bytes of words
There is text written in at least one image, not in the body of the email. In some cases there's nothing you can do, but this should be limited as much as possible. It's better to avoid these situations and use only illustrative images.
HTML: images with 1200-2600 bytes of words
Similar to the previous one, but more text was detected inside the images.
UPPERCASE_50_75
The newsletter content contains a large percentage of uppercase text, so it could be treated as spam.
UPPERCASE_75_100
Similar to the previous problem, but even more uppercase text was detected in the content.
Message is 10% to 20% HTML obfuscation
The HTML code appears to be obfuscated. If the text was copied from Microsoft Word or similar editors, some of the editor's own tags were probably copied along with it, creating suspicious text that spam filters interpret as possibly containing code, which would cause it to be treated as spam.
HTML with unbalanced body and tags
This message tells us there is an error in the email's HTML code. Some HTML tag has probably not been closed correctly. This kind of error will affect how the message is displayed.
URI_WP_HACKED_2
It means there is a WordPress URL in the newsletter that may have been compromised or hacked.
FUZZY_CREDIT
The email includes words such as "credit" in the text, along with other related words, synonyms, etc.
URI_GOOGLE_PROXY
It means the newsletter contains a link or an image loaded through Google's proxy servers (usually googleusercontent.com addresses). Spammers and phishers use these proxies to hide the real destination of their links, which is why the rule penalizes the email. To fix it, upload your images to Mailrelay's media manager and make sure your links point directly to the destination page, without shorteners or intermediate redirects.
SUBJ_ALL_CAP
It means the email's subject line is in capital letters.
FUZZY_AMBIEN
It means 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 running phishing scams, who hide a fake URL behind a link with text that looks different. For example:
Click here to update your account:
www.paypal.com
And when we click, we end up going to something like this:
http://127.1.1.1/your-data-was-hacked.html
Summary: how to avoid the spam folder and improve deliverability
The best practices and possible solutions we've discussed, as well as the error messages shown by SpamAssassin, will help you improve your email deliverability.
Of course, it isn't a 100% guaranteed method, because in the end everything always depends on third parties, the email providers; however, it's useful to know what you can do to improve your results.
That's why it's always a good idea to stop and run a few checks before sending a newsletter: make sure your domain is authenticated with SPF, DKIM and DMARC, that your list is clean, that unsubscribing is easy, and review the campaign's spam report.