Sending Emails? 



How email filters work!


The success of your email campaign is heavily dependent on email filter technology. Filters not only block incoming messages but triage them as well. Today, many email service providers break down your messages by social, commercial, newsletters, and other categories. They leverage specific criteria to evaluate an incoming message and place it in the relevant folder. 

Spam filters work in the same way and assign a spam score to the message. If the score meets a certain threshold, the email is inboxed. On the contrary, you’ll find it in the JUNK/SPAM folder. There is a long list of spam criteria that gets modified and adapted every day, so keeping up with it is a challenge. Not only that, but these practices never get published, leaving the consumers guessing at all times.

Types of spam filters

  • Content – checks the content of an incoming message for spam trigger words, malicious attachments, refined HTML code, etc.
  • Header – checks the header of an incoming message for infectious or falsified information.
  • Blacklist – checks whether an incoming message has been sent from a sender not specified in the blacklist.
  • Rule-based or heuristic – checks an incoming message according to user-defined criteria. These may include spam triggers for specific senders, words in the subject line, etc.
  • Permission – requests approval from the recipient to accept an incoming message.
  • Challenge-response – requests approval from the sender to send a message (by entering a password or another authorization method).

Spam filters can differ by not only the criteria to be assessed but also how they are implemented. Here are the most common options:

  • Gateway – checks an incoming message according to the criteria that the filter considers “spammy” based on the incoming email analysis. Usually, a gateway filter is implemented as a physical server to detect phishing, malware, viruses, and spam. 
  • Hosted – checks an incoming message after the gateway spam filter approved it. Hosted filters, also known as third-party, use content and reputation criteria to assign a spam score to the email.  


What you need to make a spam-proof email

Combating spam looks pretty simple. But each type of email filter comprises many protocols or rules to be followed. For example, header filters are not limited to the information in the headers. They take into account the reputation of a sender’s domain and IP address, check email authentication policy, and so on. Here are some ways to get ahead of the game:

Part 1 – Impeccable sender reputation 

Domain reputation

Email filters will assess the reputation of your domain based on the following metrics:

  • Complaint rate – how many emails sent from your domain have been reported by recipients as spam (by percentage). This is the most critical value for your domain reputation and deliverability in general. A high complaint rate signifies that your email marketing is unwanted – it either targets false recipients or delivers poor value. The optimum complaint rate is below 0.1%.
  • Inbox placement rate – how many messages of all emails sent from your domain have been inboxed (by percentage). This metric is more accurate than the delivery rate because it counts only inboxed emails. The optimum IPR is above 80%. 
  • Spam placement rate – how many messages of all emails sent from your domain have been trapped in spam (by percentage). Your goal is to reduce this metric as much as possible. The optimum spam placement rate is below 10%.
  • Hard bounce rate – how many messages of all emails sent from your domain have been sent back due to an invalid or non-existent recipient address (by percentage). This metric is more valuable than a soft bounce rate that counts the rejected emails due to a short-term issue (full mailbox, server down). The optimum hard bounce rate is below 2%.

Domain or email authentication

Authentication is another vital element of your domain reputation. It is a must-have thing for any legitimate sender to protect the domain against phishing and spoofing. Email authentication rests on three widely adopted standards:

  • SPF – validates whether an IP address is authorized to send emails from a particular domain
  • DKIM – authentication of emails using keys for signature-verification
  • DMARC – email authentication using SPF and DKIM standards

IP address reputation

Blacklist is a crucial metric for measuring the IP address reputation. You should not be listed in any blacklist from BRBL to Return Path. If, for some reason, you have been, make sure to launch a blacklist-removal process. The most challenging thing here is to find out the cause. Apart from that, some email filters consider other metrics like complaints, spam trap hits, rejected messages, etc. The type of IP address, dedicated or shared, is also important:

  • Dedicated – one sender is responsible for the IP address reputation.
  • Shared – multiple senders are responsible for the IP address reputation.

A new IP address is like a brand new car – it needs a break-in period. Email service providers apply restricting measures to new addresses to combat spammers. 

Tools to check the sender’s reputation

  • Sender Score by Return Path – a popular tool to assess sender reputation and find out how email service providers view an IP address.
  • IP Reputation Monitor by GlockApps – lets you check whether your IP has been blacklisted or not. Also, you can use the tool to delist your dedicated IP address. 
  • Barracuda Reputation System – a tool to check your IP address and domain based on a real-time database of IP addresses with poor and good reports.
  • Email Reputation by Cisco’s Talos – check your reputation by three score ranking: Good, Neutral, and Poor. Neutral means that messages sent from the domain or IP address may still be filtered or blocked. 

Part 2 – Polished email content

Some time ago, what you wrote in the email was the primary catch of spam email filters. They checked incoming messages for spam trigger keywords, blacklisted links, and other inappropriate elements. Today, the content check is of lower priority than the sender’s reputation, but content filters are still actionable.

Subject line

  • Avoid promotional keywords like buy/sale/discount in subject lines.
  • Words typed with all capital letters are bad manners.
  • Exclamation points are no-go!!!
  • Focus on what may trigger the recipient’s interest, like features or specs of the product/service you promote.
  • Personalized subject lines can boost the open rate by 10%.

Pay particular attention to this element of your emails. A weak subject line can still pass spam filters, but if the recipient marks your email as spam, it directly impairs your sender reputation. 

Body text

Recipients tend to distrust unreadable and grammatically poor text. Content filters will put your email campaign to spam if the number of spelling errors in your body text is too high. Always make sure to check grammar and proofread your body text. Readability is also crucial for positive recipient engagement, so mind that too.

If you combine a plain text with an image in your email campaign, keep a balanced content ratio – 60% plain text and 40% image. The thing is that spam filters may catch an email if they cannot scan the text due to large images. 


It is a regular practice for email marketers to send multi-part messages that contain both plain text and HTML. The latter lets you improve the engagement of your emails and make the content eye-catching. An HTML section that has formatting errors or broken tags is a sure way to the spam box. Always check your HTML content before sending it.


It was already mentioned that an embedded image should not exceed 40% of the total message body. Also, it might be helpful to avoid heavy images at all. As an alternative, you can compress the image and link it to your web server or any credible service. This decreases the message size and accelerates the processing and loading of the email template. 


Having attachments is a red flag for email filters. Solid email filters will put a commercial or transactional email with an attachment in spam right away. So, the best you can do is to provide a link to a particular file placed on your website or another credible location. 

Media content

Abuse of media content in your email campaign increases your message’s spamminess and reduces recipient engagement. If there is media that your email can’t do without, add a link to it. Avoid dynamic scripts – spam filters won’t let them in.  

Part 3 – Engaged recipient

If you want high deliverability, make sure your recipients are engaged. Intricate email filters do assess engagement, which comprises the following metrics:

  • Open rate – how many times the recipient has opened your email.
  • Click rate – how many times a recipient clicked on a link in your email.
  • Click-through rate – how many times a recipient engaged with at least one link in your email.
  • Conversion rate – how many recipients completed the desired conversion goal.

To increase the chances of your email hitting the recipient’s inbox, you need to take consider:

Email formatting

  • An email should open correctly in most clients and devices. Use the preview testing tools to check this.
  • An email should load fast. Avoid large images and dynamic scripts. 
  • Embedding forms are also a red flag for spam filters. It’s better to replace embedded forms with a link or a CTA button.
  • Make sure the message is free of broken links. 
  • Do not abuse colors and fonts. Email filters consider irregular font colors and sizes, as well as invisible text. Even if a message with abused text passes through spam filters, a recipient is likely to manually send it to the spam folder.

Email branding

  • Use your brand’s name in the “from” header. This reduces the spam complaints rate and increases the open rate. Also, it is a good practice to use a front person as an email campaign sender. In this case, add “from <your brand>” to the header to increase credibility. For example, Patrick Cosworth from DealerMedics.
  • You can brand other elements of your email template, including subject lines, headers, and even links. This is good for building recognition and sorting the email by folders. 
  • The design and content of your email campaign should be in line with your brand. Also, visual branding is best when it is consistent with the personas you’re aimed at. 


Follow-up is another way to earn the credibility of email service providers. When you follow up with your recipients, you show that you want to engage them. Follow-up cadence is important. Emailing your customer multiple times within short periods is suspicious and can lead to the spam folder. Pace yourself! 

Do not forget the unsubscribe link

If your emails lack an unsubscribe button/link, that’s also an excellent way to end up in the spam folder. Email service providers identify bulk emails as spam if no opt-out option is available. This is legal due to the CAN-SPAM (Controlling the Assault of Non-Solicited Pornography And Marketing) Act of 2003. If you think that you can improve the unsubscribe rate by omitting this button or link in your transactional emails, you’re making a mistake. 



There are a lot of contributing factors that will place your emails in the Junk/Spam folder. Be sure to be mindful of the information in this write-up and always put your best foot forward. It only takes one customer to mark your email as spam to start wreaking havoc on your email follow-up process.


Article credit: Zakhar Yung –