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How to avoid spam filters: getting emails to inbox

Posted on: November 4, 2020

Reading Time: 9 minutes

Category: B2B marketing

How to avoid spam filters: getting emails to inbox

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Knowing how to avoid spam filters is a crucial part of a successful email campaign, whether that’s traditional email marketing or sales prospecting. 

Some factors that affect it are obvious, some you would cover as part of best practice email marketing, some are technical and some may come as a surprise. But all of them must be considered to ensure emails land in the inbox.

Some say the Marina Trench is the least explored place on earth. The junk folder of an email account runs a close second.

What are spam filters?

Spam filters detect virus infected emails, as well as unwanted and unprompted messages (collectively known as spam or junk) and block them from arriving at your email inbox. In order to do this, spam filters monitor a large number of signals to assess which emails are genuine.

Statistics show that in 2008, 92.6% of all emails sent were classed as spam. That figure has dropped significantly, largely due to spam filters and regulations such as GDPR, but last year, spam still accounted for 28.5% of all email.

When considering spam filters, we tend to think of bad actors, genuine junk and malicious emails such as phishing attacks (disguising a fraudulent email as a trustworthy entity in an attempt to obtain sensitive information) However, it is entirely possible that your emails can get flagged as spam, even if you’re not a spammer.

Let’s dive into how to avoid spam filters in a little more detail.

How to avoid spam filters

If you are engaging in email marketing or prospecting, it is natural to ask “How do I avoid spam filters?”. Increasing delivery rates is something we know a lot about, having spent the last five years working on the problem.

The reality of email marketing today is that not reaching the inbox is a very real possibility. There are spam and junk folders, but your mail can get blocked completely and not even make it to a spam folder.

And now, Gmail adds in a secondary level, where you might have avoided being marked as junk, but still land in a category tab, with social, promotions, updates and forums all added alongside the (primary) inbox.

There are likely hundreds of factors behind your emails ending up in the spam folder. We’ve listed out the main, actionable ones below, with 22 factors to keep an eye on.

Two red mail boxes in Denmark

Types of spam filters

There are a huge number of factors built into spam filtering technology. Some are very technical, some monitor the content of your emails, and some look at issues arising before or after your mails have been sent.

The different types of filters can be grouped into the following six categories:

  • Lists and subscribers
  • Technical
  • Written content
  • Other content
  • Ethics/regulations
  • Engagement

Lists and subscriber spam filters


When a user adds an email address to their contacts or Safe Senders list, the address is added into a whitelist. This will ensure future emails from the sender bypass the spam folder.


You have to allow users to unsubscribe. It’s the rules. I know you want to keep them all, but legally, you must provide a link in the email that allows users to easily unsubscribe.

Undeliverable mail

Some of your undelivered mails may be a result of email addresses that are no longer active. If this continues for too long, or the numbers increase over time, this will begin to harm your domain reputation. This is an issue that can snowball; as your reputation takes a hit, deliverability starts to suffer even more.

Technical spam filters

Sender address

Your sender address is simply the email address you are sending messages from. If yours has numbers and letters that seem unnatural or suspicious, your email runs the risk of being flagged as spam. Basically, don’t send emails from addresses such as

DNS Blacklist

A DNS Blacklist, also known as a Realtime Blacklist, is a spam blocking list of IP addresses or domains that are known to send spam. End up on one of these lists, and your mail server won’t even send your emails. Although large ISPs have their own blacklists, many use publicly available lists maintained by specialist companies.

Email Service Provider

An email service provider (ESP) allows users to do two basic things: store email addresses and send emails. Which ESP you use will affect email deliverability rates. Reputable ESPs can refuse to accept emails sent from unknown or disreputable ESPs. 

Deliverability set up

There are also a lot of technical set up factors that you have to consider, including snazzy acronyms including SPF, rDNS, DKIM (Domain Keys Identified Mail) and MX records. If you want to read more, we’ve covered email deliverability in detail before.

Content spam filters

Subject line

We’ve covered how to write the perfect subject line before, but before you write the perfect subject line, makes sure you don’t fall for any of the below traps to avoid spam filters. 

  • Limit numbers and currency symbols
  • Use any special characters sparingly
  • Don’t shout in CAPS LOCK
  • Avoid unnecessary punctuation
  • Don’t use Re: or Fwd: prefix where there has been no previous email 

Honest subject

Pretty simple this one, the subject should reflect the content of the email. If it doesn’t, it’s dishonest and could send your email to the junk pile.

Spam copy

Of course, you want to wow people with your content and drive them to take an action of some sort. But some words cause spam filters to trigger, as they tend to be used in junk emails.

Words relating to finance, money, and winning are common spam triggers, but there are many more that can flag your email too. Take a look at these 202 email spam words.

Spelling, punctuation and grammar

We’ve all read phishing emails that are littered with typos. This is a clear sign that your email might not be authentic, but it’s also such a simple one to fix. Use a spell check, a service like Grammarly, or get a colleague to proofread.

You definitely want a link or two in an email – generally that’s how a reader takes an action. But too many and it looks spammy. Spam filters monitor text to link ratio, so you’ll want to keep the number of links to a reasonable amount.

To check the above points, try our free Email Awesomeness Checker. Get a score on your email content efforts and receive tips on improving it.

Other content filters


One of the most common mistakes is having an image to text ratio that skews too heavily toward images. Spam filters can view images, so mark them as potentially risky. And while people are visual, you should always have the copy that provides the detail, and helps avoid those spam filters.

HTML and plain text

HTML emails should have a text-only version included too, and those that don’t risk flagging spam filters. These emails are flagged it for greater scrutiny, and testing shows that the same message sent with the plain text version doesn’t go through this additional security layer.


Not adding attachments will help to avoid spam filters. Filters see them as potentially harmful, so if you want to present some collateral or a price list, direct them to the relevant page on your website.

Ethics and regulations

Misleading From field

We’ve all had emails like the one below, where a spammer attempts to cover a nonsense email address with a more plausible name in the from field. It’s classic spam.

spam email screenshot

Bought lists

Emailing bought lists returns bad results, could ruin your brand reputation, can be against certain anti-spam laws, and is arguably unethical.

Not only that but as the accuracy of static data quickly decays, many of the email addresses will no longer be used. You know what that means by now – undelivered mail and damage to your domain reputation.

Company information

Google likes company websites to include NAP (Name, Address, and Phone number), to verify who the company is and ensure their search results feature trustworthy sites. For emails, including this information does a similar job, and can help to boost your trust to ESPs.

Inactive addresses

Sending emails to inactive addresses will increase the number of undelivered mails, and trigger those pesky filters. Luckily, tools exist to tell you whether the inbox exists or is risky to send to. And they do it without actually sending an email, so your domain reputation remains intact.

Engagement spam filters

Message engagement

There are a lot of different signals that ESPs can monitor. We’ve already covered whitelisting an email address. The following signals are also taken into account, and show either positive or negative engagement:

  • Email read 
  • Email replied to
  • Marked as “not spam”
  • Marked as spam 
  • Email moved to other folders
  • Email forwarded 

Bounce rate

Bounce rate is known to anyone familiar with website analytics. Email bounce rate is the percentage of emails that get returned to you. 

Bounces fall into two categories: hard and soft. Soft bounces are generally a temporary problem, although if a message fails several times it will become a hard bounce. A hard bounce happens because either the address doesn’t exist or the recipient server has blocked delivery.

If you don’t pay attention to bounce rate and it becomes too high, it signals ESPs to lower your sending reputation, increasing the probability that your mails hit the spam filter.


The frequency with which you send your emails – something that concerns email marketing rather than email prospecting – should be monitored and optimised as part of your marketing efforts. But get it wrong, and it can have an impact on your emails hitting the spam filter. 

Send too often and you’ll increase unsubscribes and users blacklisting you – those engagement metrics we talk about. But if you don’t send often enough, users might not remember signing up and treat your email as spam. Optimal frequency varies by brand and industry, so the best approach is to test and learn.

So, just the 22 different factors to optimise and ensure you avoid spam filters. Alternatively, if you are looking for great prospecting results you can just leave it to the experts and give Sopro a call.