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How to Avoid Email Spam Filters

Spam filters work hard to reduce inbox irrelevance, so it’s important that you understand what spam is, and how spam filters and firewalls work.

The key to avoiding spam filters is understanding how they work. This guide gives you the rundown about what a spam message is, types of spam, and spam laws.

If you send enough email campaigns, you'll inevitably run into spam filter issues. According to ReturnPath, about 21% of permission-based emails sent by legitimate email marketers end up in a spam folder.

Spam filters and ISPs are working harder than ever to reduce inbox irrelevance, so it's important that you understand the definition of spam, how spam filters and firewalls work, and some of the steps you can take to avoid being flagged. Avoiding the spam folder helps you reach a larger audience, which means you have more opportunities to make sales.

At its core, spam is unsolicited, irrelevant email, sent in bulk to a list of people. This can include unsolicited commercial emails or fraudulent messages, like those including lottery scams, phishing scams or computer viruses. For example, let's say you purchased a list of email addresses from a local business organization.

On the surface, that list of addresses seems like it could contain some great prospects for your business, and you want to send them an email with a relevant offer they can't refuse. But since those people didn't give you explicit permission to contact them, sending an email to that list would be considered spam, and it may never make it to the recipient's inbox.

So, why are my emails going to spam? It's important to keep in mind that spam filters are constantly adapting to new types of spam emails, which means certain emails that weren't previously marked as spam may be sent to the spam folder based on those changes. Keeping up with email spam filters is a big part of email marketing for small business owners.

Spam laws

Mailchimp is required to enforce spam laws, not just because it's a legal obligation, and not just because it's the right thing to do, especially when it comes to unsolicited or dangerous emails. The cost of spam is high as it negatively impacts your digital communication deliverability rates, and we want to make sure your email marketing is effective and your emails reach their recipients. We have some very strict rules that must be adhered to in all countries to stop spam, but you may find that your country has additional requirements. We'll cover the laws in the United States and Canada in this guide, but you can visit our Knowledge Base for details on Mailchimp's requirements and requirements of international laws.

The CAN-SPAM Act of 2003 became law on January 1, 2004. According to the FTC, if you violate the law, you could be fined $11,000 for each offense—that's $11,000 for each email address on your list. ISPs around the country have already successfully sued spammers for millions of dollars under this law. If you send commercial email (generally sales or promotional content), you should familiarize yourself with the requirements of CAN-SPAM.

A few key points of the law include:

  • Never use deceptive headers, From names, reply-to addresses, or subject lines.
  • Always provide an unsubscribe link.
  • The unsubscribe link must work for at least 30 days after sending.
  • You must include your physical mailing address.

Canada's Anti-Spam Legislation (CASL) went into effect on July 1, 2014 and carries penalties of $1-10M per violation. CASL is very similar to the CAN-SPAM Act, but has some minor differences and covers all electronic messages, not just email. This article details the basics of CASL

Spam negatively impacts deliverability rates, and we want to make sure your emails reach their recipients.

Why are my emails going to spam?

As a small business owner, your email marketing efforts can be seriously affected if your marketing emails end up in people's spam folders. Understanding how spam filters and firewalls work is an essential part of making sure your emails end up in people's inboxes instead of their spam folders.

Your emails may end up being marked as spam as a result of recipients frequently marking them as spam. Spam filters adapt to users' preferences, so several people marking your emails as spam can lead to your emails ending up in the spam folder. You can take steps to avoid people marking your emails as spam, including making sure you're not sending too many marketing emails to people on your mailing list.

In some cases, your emails may end up in the spam folder because of the content of the email or the subject line. There are certain phrases that may trigger spam filters, which are typically the types of phrases that email spammers use. Avoiding these spam triggers can help keep your emails out of the spam folder, but it can also help keep you from looking like a spammer when you're dealing with customers.

Sometimes emails are marked as spam because you don't have permission to send emails to that recipient. You must obtain express permission from email recipients if you want to send them marketing emails, so purchasing an email list or sending emails to unauthorized addresses can lead to spam triggers. The best thing you can do for your email marketing campaign is take the time to build a mailing list organically, that way your email recipients are interested in your emails and they're not marked as spam.

Emails can also be marked as spam because you didn't include an unsubscribe link, you included too many attachments, or you haven't set up email authentication yet. Regardless, it's your job as a small business owner to figure out why your messages are ending up in the spam folder and put a stop to it.

What is spam complaint rate?

The spam complaint rate measures the number of times people report an email as unwanted or spam compared to the total number of emails sent. In simpler terms, it shows how many recipients didn't like an email and flagged it as spam out of all the emails that were sent.

Having a lot of spam complaints in a short period of time can negatively impact email deliverability and reputation, potentially leading to your emails getting blocked or immediately sent to the spam folder. Keeping the spam complaint rate low helps you maintain a positive sender reputation and ensures that emails reach the intended recipients' inboxes.

Spam filters

Each email service provider offers spam filters. Spam filters consider a long list of criteria when judging the “spamminess” of an email with the goal of reducing spam. They'll weigh each factor of the spam you receive or send and add them up to assign a spam score, which helps determine whether a campaign will pass through the filter. If the score exceeds a certain threshold, your email will get flagged as spam and go straight to the junk folder.

Each spam filter functions a bit differently, though, and “passing” scores are typically determined by individual server administrators. This means that an email could pass through Spam Filter A without issue, but get flagged by Spam Filter B.

As for that list of spammy criteria, it's constantly growing and adapting, based on—at least in part—what people identify as spam with the ‘Mark as spam' or ‘This is junk' button in their inbox. Spam filters even sync up with each other to share what they've learned. There's no magic formula—and spam filters don't publish details regarding their filtering practices—but there are steps you can take to avoid landing in your subscribers' junk mail folder.

Campaign metadata: Spam filters want to know that you're acquainted with the person receiving the email. We recommend using merge tags to personalize the To: field of your campaign, sending through verified domains, and asking recipients to add you to their address book.

Your IP address: Some spam filters will flag a campaign if anyone with the same IP has sent spam in the past. When you send through Mailchimp, your email is delivered through our servers, so if one person sends spam, it could affect deliverability for our other users. That's why we work vigilantly to keep our sending reputation intact, and it's important that all users abide by our Terms of Use.

Coding in your campaign: Spam filters can be triggered by sloppy code, extra tags, or code pulled in from Microsoft Word. We recommend using one of our templates or working with a designer.

Content and formatting: Some spam filters will flag emails based on specific content or images they contain, but there's not an all-encompassing set of best practices to follow or things you absolutely need to avoid. But, we do have a few recommendations:

  • Design your campaign to be clear, balanced, and to promote engagement from your subscribers.
  • Make sure your subscribers have opted-in to receiving your emails.
  • Be consistent. Try not to stray too far from the content and design that your audience already associates with your brand, website, or social media channels.
  • Test, test, test! Use A/B or multivariate testing to learn how changes with your content affects delivery and engagement.

Subscriber engagement: Spam filters take email engagement into consideration when deciding whether something gets flagged as spam. If your emails tend to receive low engagement rates, you might find that more of your messages land in the spam folder. To increase engagement—and boost the relevance of your campaigns—we recommend using personalization and segmentation to tailor your marketing to the right audiences.

Spam filters want to know that you're acquainted with the person receiving the email. We recommend using merge tags to personalize the To: field of your campaign.

Email firewalls

Firewalls also help to fight against spam—among other internet security areas—and are a lot like spam filters in that they are designed to regulate incoming email based on a set of rules that have been established by the email server with the goal of reducing the amount of spam you receive.

Think of them as gatekeepers, especially useful when there are high volumes of spam. They're used by ISPs, large corporations, and small businesses, and they all communicate with one another to help identify spamming techniques, detect spam, and stop spammers.

But how do firewalls know what spam is? Your own recipients teach it. When you send an email to your list, and someone on your list thinks it's spam—or doesn't remember opting-in to your list, or if you never had permission in the first place—that recipient can report you. Mailchimp's staff receives copies of any complaints that come in, so we can disable the sender's account and investigate immediately.

Firewalls rely on reputation scores to block emails before they even get to the content-based spam filters, and they all calculate sending reputation differently. Once you've been reported, you'll remain on the radar of these firewalls; this helps prevent someone from switching between different email servers to send more junk after being reported. These gatekeepers will know to block all emails with your name in it from now on, no matter who sends it or where it comes from.

How do firewalls know what spam is? Your own recipients teach it.


Omnivore is Mailchimp's abuse-prevention algorithm that keeps our system clean by predicting bad behavior in a campaign before it even gets out the door. Spam filters are equipped to catch obvious and evil spam, but they're not as effective at predicting permission issues. ESPs often have a hard time detecting ignorant spammers, too. Omnivore can predict bounces and lack of permission within a user's list and send a warning to help them develop better practices before it's too late.

If you're not a spammer, how does Omnivore affect you? Well, because it prevents abuse on such a massive scale, you'll achieve better deliverability by default. Even problem-free senders benefit from a self-cleaning system.

Omnivore can predict bounces and lack of permission within a user's list and send a warning to help them develop better practices before it's too late.

Abuse reports

When people receive email that they think is spam, they can just click a button in their email client to label it as such. In most cases, once that button has been clicked, an abuse report is created and sent to the recipient's email program, email clients, or ISP. If enough of these reports are received, an automated warning message will be sent to the sender.

When you use Mailchimp, an abuse complaint will be generated each time someone marks your campaign as spam, thanks to the feedback loop in place for most ISPs. We'll immediately remove that recipient's email address from your active list and into the abuse complaints area of your account.

Once abuse complaints reach our threshold, you will receive a warning from our abuse team. If the complaint rates exceed that threshold, your account will be suspended, and our team will need to conduct an investigation into your list collection process.

High levels of spam and abuse from a user can result in the IP addresses being denylisted by ISPs and anti-spam organizations. And, if you use Mailchimp for sending—or any email marketing service, for that matter—that means your emails can affect the deliverability of hundreds of thousands of other legitimate marketers. It's very serious—one bad apple can truly spoil the whole bunch.

That's why we've developed Omnivore; we're constantly monitoring incoming complaints, and we have a team of human reviewers that review Mailchimp accounts.

Accidental abuse reports

You don't have to be a spammer to get reported for spamming. Even legitimate marketers who only use opt-in lists can have their email reported as spam, even if it's not. Sometimes it's a simple mistake, like when an user clicks the spam button to unsubscribe from an email.

Since it's almost inevitable that you'll receive complaints every now and then, Mailchimp is constantly monitoring abuse reports from ISPs, denylists, and anti-spam networks, so we can immediately pinpoint problems as they arise and investigate the account in question.

The most common reasons emails end up in a recipient's spam folder are:

Blacklisted IP addresses

When an email is sent from an IP address or sending domain that is known for sending spam or has been flagged for suspicious activity, email providers may automatically route those emails to the spam folder.

This can happen if spam detection systems have blacklisted the IP address. To avoid this, businesses must ensure that their IP addresses are reputable and not associated with spamming.

Poor list management practices

Sending emails to recipients who haven't opted in or unsubscribed from mailing lists can lead to emails being marked as spam. Additionally, sending emails to outdated or inactive email addresses can also trigger spam filters.

Maintaining clean and up-to-date email lists, obtaining proper recipient consent, and honoring unsubscribe requests are essential to avoid spam complaints and improve email deliverability.

Misleading subject lines

Subject lines that are deceptive, misleading, or overly promotional can raise red flags for spam filters and cause emails to be flagged as spam. This includes subject lines that use all capital letters, excessive punctuation, or spam words such as "Act Now!" or "Free Offer!"

Crafting clear, relevant, and concise subject lines that accurately reflect the content of the email can help improve deliverability.

How spam filters work

Most email service providers that use spam filters employ techniques to analyze incoming emails and determine whether or not to deliver them to inboxes, mark them as spam, or block them altogether. If you send spam, you simply won't get the results you're looking for. Here's how spam folders work:

Content analysis

The filter evaluates the content of emails to assess their likelihood of being actual spam versus legitimate emails. This analysis includes scanning for spammy keywords, suspicious URLs, and other indicators commonly associated with unsolicited or malicious emails.

Spam filters can identify and flag emails that exhibit spammy characteristics by examining factors such as subject lines, body text, and HTML code.

Sender reputation

Spam filters also consider the reputation of the sender when assessing the legitimacy of an email. Sender reputation is based on the sender's IP address, domain reputation, and past sending behavior.

Email senders with a positive reputation, who consistently send relevant and engaging content to recipients who want to receive it, are more likely to have their emails delivered to the inbox.

Conversely, senders with a poor reputation, such as those associated with spamming activities or frequent spam complaints, may find their emails filtered or blocked by spam filters.

Engagement metrics

Spam filters also consider engagement metrics to determine an email's relevance and value to recipients. Open rates, click-through rates (CTRs), and unsubscribe rates tell ESPs whether your recipients are satisfied with your email content.

Emails that receive high engagement signals, such as opens and clicks, are more likely to be considered legitimate and delivered to the inbox. Conversely, low engagement rates may indicate an email is unwanted or irrelevant, increasing the likelihood of being marked as spam.

Email design and content

Your email design and content can signal to inbox providers whether your emails are spam or not. Remember, your goal shouldn't be to trick spam checkers.

Instead, businesses should ensure spam filters don't impact their ability to send legitimate messages to customers and subscribers. Here are a few tips to improve your email and design content:

Subject lines that avoid spam triggers

Your subject line is the preview of your email. It's the first element recipients see when they receive an email, making it a critical factor in whether they open or ignore the email.

Subject lines should be clear, relevant, and free of spammy language or gimmicks to avoid triggering spam filters and ensure deliverability.

Crafting engaging and compliant content

The content of an email should provide value to recipients while adhering to legal and ethical standards for email marketing. This includes offering relevant information, promotions, or offers tailored to the recipient's interests and preferences.

Additionally, emails should include clear and conspicuous unsubscribe links, as required by anti-spam legislation such as the CAN-SPAM Act in the United States.

Balancing text and images

While images can enhance an email's aesthetic appeal and convey information visually, they should be used strategically and accompanied by descriptive alt text.

Over-reliance on images without sufficient text content can trigger spam filters and make emails inaccessible to recipients who have images disabled by default.

Avoiding spammy language and phrases

Using spammy language or phrases in email content can trigger spam filters. Common spammy language includes words like "free" and "guaranteed" or excessive use of exclamation marks and all capital letters.

Instead, focus on using clear, concise, and relevant language that resonates with your target audience and avoids sounding overly promotional.

Consistent email deliverability is essential for the success of email marketing campaigns, ensuring that messages reach recipients' inboxes reliably and effectively.

To maintain high deliverability rates, businesses must proactively manage their email lists, engage subscribers, and monitor key metrics. Follow these tips to ensure your emails reach subscriber inboxes:

Regularly cleaning your email list

Removing inactive or invalid email addresses ensures your messages reach engaged and interested recipients. This helps improve deliverability by reducing the likelihood of emails bouncing or being automatically marked as spam.

Re-engaging inactive subscribers

Re-engaging inactive subscribers is an effective strategy for revitalizing your email list and improving engagement rates.

This can include sending targeted re-engagement campaigns, offering incentives or promotions to encourage activity, or simply reaching out to subscribers with personalized messages to gauge their interest.

Monitoring bounce rates

Monitoring bounce rates is crucial for identifying and addressing potential deliverability issues. Bounce rates indicate the percentage of emails returned as undeliverable, either due to invalid email addresses (hard bounces) or temporary issues with the recipient's mailbox (soft bounces).

High bounce rates can negatively impact sender reputation and deliverability, so it's important to monitor and address any issues that arise regularly.

Using reputable email service providers

Reputable ESPs have established relationships with internet service providers and email clients, which can help ensure that your marketing emails are delivered.

Additionally, reputable ESPs offer advanced deliverability features, such as authentication protocols, inbox placement optimization, and dedicated IP addresses, to maximize deliverability and inbox placement.

Choosing a trusted ESP like Mailchimp with a track record of delivering results enhances email deliverability and drives better outcomes from their email marketing efforts.

Avoid becoming an email spammer

When you're running a small business, the last thing you want to do is alienate potential customers by coming across as an email spammer. Email marketing can be difficult, but doing what you can to avoid ending up in recipients' spam folders is a good start. As long as your emails are making it to people's inboxes, you have a shot at converting email addresses to sales.

If you want to avoid becoming an email spammer, you need to understand the basics of spam filters, email firewalls, and other potential spam triggers. There are rules you must adhere to as a small business owner, and failing to adhere to these rules can make it hard to maximize your email marketing campaign. If you're not including an unsubscribe link or you're using confusing “From” information, your email may be marked as spam before it ever reaches people's inboxes.

You can also keep your messages from landing in the spam folder by making sure you're not sending too many messages. It's okay to update customers about new products you're developing or sales you have going on, but you don't need to send out several marketing emails every week to get your point across. The more you flood people's inboxes with marketing emails, the more likely those people are to mark your emails as spam.

The good news is that Mailchimp can help you take control of your email marketing campaign to get the most out of your time and money. With Mailchimp, you get access to powerful email marketing tools that can help you give your marketing campaign a boost, as well as website development and other marketing tools. If you need help making sure your emails are arriving in recipients' inboxes, try Mailchimp today to see how we can make a difference.

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