6. Today's Spam Filters & How They Work

What Is A Spam Filter?

Spam filters are programs that identify unwanted or unsolicited emails and prevent them from reaching inboxes. Major ISPs like Google and Yahoo have built-in spam filters and filtering processes to ensure their users are protected from “junk” email.


How Spam Filters Identify Spam

Spam filters identify spam based on a long list of criteria:

  • Relationship with subscriber
  • Reputation of IP address and sender domain
  • Quality of email subject line, teaser, and content
  • Quality and safety of links in email
  • Presence or absence of images
  • Ratio of images to text and links to text

Spam Filters Make Judgements Based On Your Actions - But Also On The Actions Of Your Subscribers

Using Bayesian Filters (strongly recommend you take a quick look at the description by clicking that link) as a means of making judgements regarding how to process an email is now common place, and the judgements are based on FAR more than you will probably have ever imagined. Understanding how they work should totally change your understanding of what is going on out there in the emailing jungle, and make you rethink in particular how you test emails, as you will probably have been doing yourself a dis-service in the past.

Few people realise that using those Filters but Spam filters track the following subscriber actions:

  • If subscriber opens email or not
  • The amount of time subscriber takes to read email
  • If subscriber scrolls to bottom of email or not
  • If subscriber enables images
  • If the subscriber clicked a link
  • If subscriber flags email as spam
  • What tags subscriber applies to email
  • What folders subscriber adds email to
  • If subscriber forwards email
  • The number of times subscriber opens email

Tracking subscriber actions gives spam filters valuable data to shape their complex ranking formulas. Spam filters use these formulas, to determine an email spam score that is unique for each email to every individual subscriber.


This means that the same email for two different people could be ranked differently, one being routed to the Spam folder and one to the Inbox. Spam filters rely heavily on subscriber engagement to learn how to become better at identifying what is and isn’t spam for each individual subscriber.


This is making the quality of your mailing Lists evermore important, as with a bad List you are never going to avoid the Spam Filters no matter how good your email is, or how nimble our scripts are at avoiding them.


How Spam Filters Deal With Spam

Spam filters use the spam score to determine what action to take next. An email with a passing score moves to the subscriber’s Inbox. If an email is flagged as spam, filters generally take one of the following actions:

  • Send email to spam folder
  • Block or bounce email for policy reasons
  • Slow delivery rate from sender (stuck in a tar pit)
  • Block email from sender
  • Block email from IP address (effecting all other SendReach Users)
  • Block all emails from a range of IP addresses (effecting all other SendReach Users)

Big ISPs such as Google are quite good at identifying spam on a case-by-case basis and sending it to the spam folder. But no spam filter is perfect. It is impossible to guarantee that every opt-in email will make it to the inbox of each subscriber. However, there are steps we can take to give our email a good chance of reaching our audience.


Produce quality email content.

The best way to get past spam filters is to send high quality email to people who know you and expect to hear from you. If you give people what they want and expect, you can expect healthy subscriber engagement. You also want to take care of the following:


  1. Clean coding. Create your email in the SEndReach Editor to avoid superfluous HTML code which the Spam Filters sieze on immediately as a clear sign of Spam
  2. Professional formatting. CAPS LOCK, dozens of exclamation points, and wild colors should be avoided. FOR SURE!!!!
  3. Wise words. Avoid spammy words and phrases. Don’t mention getting rich quick or making tons of money or getting free everything today.
  4. Healthy linking. Don’t send emails with 40 links and minimal text. This is a popular tactic by spammers. Keep a balanced text to link ratio.
  5. Balanced images. Spammers often use images to send ‘special offers’ because spam filters can’t read them easily. Emails that contain a large image and very little text are often filtered as spam. Keep a balanced text to image ratio.

Read and understand the CAN-SPAM Act.

The CAN-SPAM Act is a US federal law passed in 2003 that sets the rules for commercial email. With fines as high as $16,000 for every email that violates the law, it’s worth understanding.


Below are a few general tips on how to comply with the CAN-SPAM Act:


  1. Do not deceive. Never use deceptive headers, subject lines, reply-to addresses, from-names, or call-to-actions.
  2. Make unsubscribing easy. Include an unsubscribe link, which must stay live for at least 30 days after sending. Remove unsubscribes within 10 days of opt-out. (At ExpressPigeon, we honor instant unsubscribes.)
  3. Include your real mailing address. You must include your actual physical postal address in your email.
  4. Never send without consent. Use our Optin Forms and Buttons to help people subscribe.

To learn more, visit the FTC website.

Grow your list ethically and continually.

Misleading or deceitful list-building tactics can crush your deliverability. Why?


People who don’t explicitly give you permission to receive your emails don’t want to hear from you. They are likely to mark your email as spam.


A few examples of what not to do:

  • Use shady offline list-building tactics. People who drop a business card in a fishbowl or sign up for a chance to win a trade show giveaway do not necessarily want email from you. Be clear when collecting emails. There are healthy ways to build an email list offline.
  • Buy an email list. It’s a really bad idea to purchase or rent an email list. Not only is it illegal and punishable by US law, it can crush your email marketing program and do long-term harm to your brand.
  • Scrape websites for email addresses. It goes without saying that this will only get you in trouble. Do this, and you’ll probably end up on blacklists.

To get past spam filters, you want to attract people who are truly interested in your email.


Design professional-looking emails.

If your emails look cheap or unprofessional, you can come across like a spammer.


Create relevant and engaging subject lines.

Spam filters and your audience don’t want misleading subject lines. Make sure your subject line engages subscribers and tells them what your email is really about.


Avoid spammy phrases like “click here” or “open now” or “act today” or “limited offer”. Also, don’t use ALL CAPS or too many numbers or special characters.


Personalize your email.

The more you personalize, the better. The least you can do is address your subscribers by their names. Use the following salutation to do so:

Hi [first_name],

Subscribers appreciate receiving an email with a personal touch. The better your subscribers know you and trust you, the less likely they are to flag your emails as spam.


Use descriptive text instead of URLs as link text.

Spam filters try to block phishing attacks, where attackers encourage readers to click on a well-known text URL that links to a different URL (attacker website). For example, a victim of a phishing attack would see “” in an email but upon clicking the link, they would be directed to “”.

Because of this shady tactic, you should avoid using URLs as link text. Instead, use descriptive text such as Top Restaurant In Town with the underlying link inserted using the "Insert Hyperlink" function.


Be consistent with send frequency.

If you don’t send emails for long periods of time, and suddenly send, subscribers may forget about you and mark the email as spam. If you’re sending too often, subscribers will get irritated and ignore you completely.

Finding the right frequency for your email program requires testing, but consistency is key. Once subscribers know how often you send, they expect to receive emails at the same rate moving forward.

When you meet their expectations, subscribers are more likely to become engaged readers and less likely to mark your emails as spam.


Avoid spam traps. 

If an email address hasn’t been used in years, ISPs sometimes convert that email address into a spam trap, called a honeypot. You will never know if an email address is a spam trap.

The best way to avoid spam traps is to never scrape websites or buy rented lists. As long as you follow ethical list-building tactics, you should be fine.

We recommend running the "remove unopens" function every month set to remove those who have not opened an email from you in the previous 6 months to make sure you have a healthy, engaged list. If your response rates are particularly low then run it to remov those who have not responded over last 3 months.


Please see for more detail on testing Broadcasts and using Seed Addresses.


Getting past spam filters is a complicated process, but it fundementally comes down to sending useful email to an interested audience using a reputable email service provider. Beware of anyone who tells you otherwise. They might be out to spam you!

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