If you send enough email campaigns, you’ll inevitably run into spam filter issues. According to ReturnPath, only about 79% of permission-based emails sent by legitimate email marketers reach the inbox. Spam filters and ISPs are working harder than ever to reduce inbox irrelevance, so it’s important that you understand what spam is, how spam filters and firewalls work, and some of the steps you can take to avoid being flagged.

What is Spam?

There are a variety of definitions and interpretations of the word itself, but at its core, spam is unsolicited, irrelevant email, sent in bulk to a list of people. 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

As an ESP, we are required to enforce spam laws, not just because it's a legal obligation, and not just because it's the right thing to do. Spam negatively impacts deliverability rates, and we want to make sure your emails reach their recipients. We have some very strict rules that must be adhered to in all countries, 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 please refer to this article for details on MailChimp’s requirements and requirements of the laws in place internationally. If you have any questions regarding the details of the laws or any potential legal ramifications, we encourage to you to consult an attorney who is familiar with this topic.

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 a lot of money for each offense—that means you'll incur a fine 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:

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 Filters

Spam filters consider a long list of criteria when judging the spamminess of an email. They’ll weigh each factor 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 folder.

Email Firewalls

Firewalls 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. Think of them as gatekeepers. They’re used by ISPs, large corporations, and small businesses alike, and they all communicate with one another to help identify spam and stop spammers. But how does this server 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 to SenderBase, the world’s largest email monitoring network. Your ESP should be registered at SenderBase, so they can properly investigate every complaint generated in response to their users’ campaigns. 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.


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 lack of permission within a user’s list and send a warning to help them develop better practices before it’s too late.

How it works

Each time you import new addresses or send a campaign to new recipients, Omnivore will scan for addresses that have never received campaigns from your MailChimp account. Then, it checks to see how many of those addresses are likely to be spamtraps, or to generate abuse complaints and hard bounces. Omnivore is very accurate in predicting list performance, and uses an algorithm to assign a level of risk to a group of addresses. When that level of risk is determined to be too high, we’ll prevent you from sending to them. If your list gets flagged by Omnivore, we recommend looking both into the collection methods and, if applicable, prior methods of management. In many cases, you may just need to reconfirm the affected list before sending can be resumed.

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

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 send 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 blacklisted 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, ESPs like MailChimp are constantly monitoring abuse reports from ISPs, blacklists, and anti-spam networks, so we can immediately pinpoint problems as they arise and investigate the account in question.

Every major ISP cares about reducing unwanted email for their customers, so when you receive an abuse report, there’s no negotiating; you’re guilty until proven innocent. As long as your email list has been collected legitimately and you are able to prove without a doubt that any complaint you received is a simple mistake, you’re in the clear. But if we have reason to question your list collection practices, your account will be disabled—or shut down altogether.

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. The following tips can help you prevent spam complaints as you start sending email to subscribers:

Education and Support

Thanks for taking the time to learn how you can avoid spam filters when sending with MailChimp. If you have any questions that were not addressed in this guide, review our collection of Knowledge Base articles or feel free to get in touch with our team.