At some point, I’m willing to bet you’ve found yourself in one (or more) of these situations:
- You sent an email to someone, but never heard back. When you talked to them about it, they found your message in a spam folder—or else, you’re forced to assume that you went to spam and you never hear from them again.
- You looked like a jackass because you failed to respond to a boss, client, or colleague, only to find later that their email ended up in your spam folder.
- You launched an email marketing campaign, but are struggling because you’re getting red flagged as spam.
Spam filters are there to protect us, and we have some manual control over what goes to spam as well. Ultimately, it’s a good thing, since there’s something like 54 billion actual spam messages sent every day. But occasionally, spam filters get in the way.
So how do spam filters work, and why do emails go to spam? If you learn the answers, you’ll be better able to monitor your own spam folder, and less likely to get trapped in those of others.
How Emails Are Sent (and How Spam Works)
Let’s go over some background information first. We use the term “spam” colloquially to refer to any unwanted email, but technically, spam refers to unavoidable, repetitive messages sent in bulk. They’re irrefutably a bad thing, because they’re irrelevant to users, they often contain viruses or scams, and they put an increased burden on ISPs and email hosting providers.
Emails are drafted in a mail client—like Gmail—and then sent out via an SMTP server. You can think of the SMTP server as functioning like a post office, accepting your mail and figuring out where it’s supposed to go.
The SMTP server communicates with a DNS server, which parses the domain name attached to the email addresses you’re sending to. From there, the message is sent to the target domain’s mail exchange server (a Mail Transfer Agent, or MTA). Then, your recipient will accept the message, usually by using a POP or IMAP client.
Spam filters generally occur on the recipient’s end. There are a few major types of spam filters:
- Gateway spam filters. As the name implies, gateway spam filters exist at the “gateway” between the MTA and a given user. Any mail attempting to reach a given domain must pass through this gateway, meeting a series of criteria. If it fails any automated checks, it gets rejected, and never reaches the user. Depending on the service you’re using, these messages may or may not end up in your spam folder; egregious messages typically never get seen.
- Third party filters. Some companies specialize in creating and managing in-depth spam filters, designed to distinguish between legitimate and unwanted messages. These can manifest alongside the gateway spam filter, or provide a secondary layer of protection, filtering messages after they get through the gateway. Depending on the type of message and the third party filter in question, the message may end up in a spam folder, a quarantine folder, or oblivion.
- Desktop spam filters. Some spam filters are available in the email client directly, and are highly customizable by individuals. Gmail and Outlook, for example, both have desktop-level spam controls, so you can automatically send certain messages to spam and/or whitelist senders who have been sent to spam in the past.
How do these filters tell what is spam and what isn’t?
We’ll get into more specifics in the next section, but generally, these filters are applied with one or more of the following parameters in mind:
- Content. Certain message content, like suspicious links, will automatically trigger a spam flag.
- Headers. Falsified information in email headers can also make an email seem like spam.
- Blacklists. Some domains and senders have been “blacklisted” as known spammers.
- Rules. User-defined rules may block certain types of emails, or those that contain certain types of content.
- Permissions. Conversely, pre-approved senders may be able to send emails, regardless of their message content.
- Verifications. Some spam filters require the transmission of a certain code to verify that the sender isn’t a spammer.
Obviously, triggering a spam filter will flag a message as spam, and either prevent it from being sent or send it to a spam folder. But what, exactly, is responsible for these triggers?
Why Emails Go to Spam
These are some of the most common reasons why emails go to spam:
1. Spam trigger words.
Lots of spam filters will activate when they see too many “trigger words” out of context within the subject line or body content of an email. These words and phrases are well-established as indicators of schemes or nefarious intentions. Some of these trigger words include:
- act now
- all natural
- compare rates
- incredible deal
- limited time offer
- cancel at any time
- check or money order
- great offer
- dear friend
- for only (usually followed by a dollar amount)
- increase sales
- order now
- promise you
- special promotion
- click here
- and my personal favorite, “this is not spam.”
If you use lots of capital letters or exclamation points, you’ll make matters worse for yourself. Of course, some of these have legitimate uses (especially in sales), so you might end up triggering a spam filter by mistake.
2. A bad IP address.
After repeated offenses, an entire IP address may end up blacklisted. And since IP addresses are sometimes reassigned or recycled, there’s a chance you may end up with an IP address that was previously blacklisted.
If your IP address is flagged as a source of spam, whether the offenses were recent or not, you could end up in all your recipients’ spam folders. Over time, these flags may disappear, but you’re usually better off getting another address or making an appeal.
3. Low engagement rates.
If you’re mass marketing via email, you may start getting flagged as a spammer if your engagement rates are too low. If you send thousands of emails, but nobody seems to be opening your message or clicking your links, it’s a sign that something is wrong.
Eventually, a gateway spam filter or third party filter will step in and take action.
4. High rates of manual spam flags.
If you’re constantly getting emails from a given source, you have the power to manually “flag” it as spam. As a first line of action, this will forward the sender’s future emails to you to the spam folder automatically.
If too many users in your sending list manually mark your messages as spam, you could become blacklisted. Don’t worry about one or two flags—you’ll need to meet certain thresholds before any organization takes action against you.
5. Inactive mailboxes.
If you’re mass-sending emails, you may end up in a spam folder if you send to too many inactive inboxes. Oftentimes, when you buy a list of email addresses (a marketing strategy that isn’t recommended), a large percentage of those email addresses will be inactive, outdated, unverified, or some combination of the three.
If you’re caught sending to too many inboxes that are no longer relevant, it’s a sign that you aren’t following best practices.
6. Misleading or sensational subject lines.
Subject lines are arguably the most important part of any email (and we’ve written about how to improve them). Accordingly, spammers often attempt to manipulate, fool, or mislead their recipients with sensational or false subject lines.
If you include false or deceitful information in your subject field, it could trigger a spam flag. For example, ALL CAPS SUBJECT LINES are a spam trigger, as are characters like $, @, # &, and excessive use of exclamation marks.
7. High send volume.
Depending on the credibility of your domain and the type of mail service you’re using, there may be limitations on how many emails you can send within a given period of time.
If you send out 100,000 emails in a single batch, and you don’t have much history as an email sender, you could be flagged as a spammer.
Sending the same message, or slight variants of the same message, over and over again will also likely trigger a spam filter; even if it doesn’t, it probably won’t help you get conversions.
Make sure you switch things up if you’re sending sales or marketing emails. Other types of emails won’t typically encounter this problem.
9. Excessive images or links.
Spammers like to include lots of links and images in their messages, as well as downloadable attachments. This is especially common among cybercriminals, who want to fool recipients into downloading a virus or visiting a certain site.
There’s nothing wrong with a handful of images and links—in fact, your marketing campaign will benefit from it. However, if you violate norms, you’ll likely trigger a spam filter.
10. Inaccurate “From” information.
The CAN-SPAM Act imposed some new rules for how marketers and companies can email individuals. If you fail to follow any of these rules, you could end up marked as spam. Even worse, if you’re caught in violation of the CAN-SPAM Act, you could face steep fines as a penalty.
One such requirement is including accurate “From” information; if you’re caught pretending to send an email from another source, like eBay, you could be considered spam.
11. Missing contact information.
The CAN-SPAM Act also requires you include contact information, including a physical address, for your business within the email. If you don’t, you guessed it, you could be considered spam.
Keep in mind, you’ll also need to provide your email recipients with some way to unsubscribe—and you must honor all unsubscribe requests in a timely manner.
12. Bad HTML formatting.
Sometimes, bad HTML formatting, or deviations from other best practices can trigger a spam filter. Spam filters are always looking for suspicious or unusual features, so there’s a chance even simple errors could be seen as spam.
13. Manual user settings.
Remember, almost everyone has the power to establish manual spam settings of their own. In Gmail, for example, it’s possible to set up automatic filters so that emails from certain senders, or those with certain keywords and phrases, are automatically sent to Spam (or another folder of your choosing).
If only one person in your contact network is having issues with your messages going to spam, and everyone else gets your emails in their inbox, a manual spam filter is likely to blame.
Keeping Better Email Metrics
Now, you should be able to answer the question “why do my emails go to spam?” Your next step should be to understand your email habits and email marketing metrics. For that, you’ll need a suite of tools to help you manage your submissions (and measure your results). If you use Gmail, one of the best tools to use is EmailAnalytics.
Once integrated with your Gmail account (a process that can be initiated with a single click), you’ll have access to a wide variety of metrics associated with your campaign. Among these are your top senders and recipients, your average response time, and even your busiest days and times. Sign up for a free trial today, and find out what you can do to improve!
Jayson is a long-time columnist for Forbes, Entrepreneur, BusinessInsider, Inc.com, and various other major media publications, where he has authored over 1,000 articles since 2012, covering technology, marketing, and entrepreneurship. He keynoted the 2013 MarketingProfs University, and won the “Entrepreneur Blogger of the Year” award in 2015 from the Oxford Center for Entrepreneurs. In 2010, he founded a marketing agency that appeared on the Inc. 5000 before exiting it in January of 2019, and he is now the CEO of EmailAnalytics.