How to Avoid Email Spam Filters

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.

What is Spam?

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.

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. 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.

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 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.

Tips and Best Practices

In email marketing, it’s important to remember that permission is key. Without permission, you could be reported for abuse whether or not you’re a legitimate marketer. These tips can help you prevent spam complaints as you start sending email to subscribers:

Choose your opt-in method wisely

Mailchimp’s standard signup forms, by default, use the double opt-in method. Double opt-in is valuable because you’ll know (and have proof) that each and every recipient gave you permission to send them emails.

But there are a number of other popular signup methods (API, integrations, etc) that allow for single opt-in, and we certainly are not discounting the validity of those, either. Ultimately, the most important thing is that your recipients give you permission to email them. You’ll need to consider your audience and the applicable legal requirements in your area to determine which opt-in method is right for you.

Don’t use purchased, rented, or scraped lists

Not only are they against our Terms of Use and notorious for providing bad addresses that lead to high bounce rates and denylisting, they don’t actually help you grow your business. Sending to a list that hasn’t given you express permission can impact your ability to market your business, potentially damage your brand, and even result in legal ramifications. Instead, grow your list organically.

Don’t assume you have permission

Even if your intended recipients are already your customers (or your colleagues, or people you met at a trade show, etc), do not send promotional email without getting permission first. Consider adding a signup form to your website or giving customers the option to sign up for your list when they make a purchase from your store.

Set expectations when people join your list

If your subscribers think they’re signing up for monthly newsletters and you start sending them weekly promotions, they might not be subscribers for much longer. Tell people what you’ll be sending and how often you’ll be sending it. If you want to send out different content (monthly newsletters, weekly special offers, etc.), consider setting up groups in your list so subscribers can choose what content they want to receive from you.

Don’t wait too long before contacting your subscribers

Every mailing list can go stale if it’s not used regularly, even if subscribers were originally collected via double opt-in. Lists with a lot of stale addresses can lead to high rates of bounces, spam complaints, and unsubscribes.

In addition to keeping an up to date permission reminder in each campaign, consider setting up a process where new subscribers receive emails from you right away, perhaps through a welcome email sent with Mailchimp’s automation features.

If you’re worried that your list has gone stale, we recommend removing the list and reconfirming outside of Mailchimp.

Treat your email campaigns as an extension of your brand

Your customers probably already have an idea of what type of content, imagery, and design elements to expect from you, so don’t stray too far and risk harming that recognition factor. If you have any questions about what content, designs, or subject lines your customers will respond to and engage with, don’t just leave it to chance—use Mailchimp’s testing features to find out.

Don’t hide the unsubscribe/opt-out link in your campaigns. Mailchimp (and the CAN-SPAM Act) requires that an unsubscribe link be present in every campaign that you send. When the link is prominent, people who no longer wish to receive your emails will be able to quickly and easily remove themselves from your mailing list. When the link is hard to find, the recipient might be more inclined to mark your message as spam, resulting in an abuse complaint within your Mailchimp account.

Not only are purchased or scraped lists notorious for providing bad addresses that lead to high bounce rates and denylisting, they don’t actually help you grow your business.

Education and Support

If you have any questions that were not addressed in this guide, review our collection of Help articles or feel free to get in touch with our team.

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