How Spam Filters Work (And How You Can Avoid Them)

Screen detail showing large amount of Spam mail.Having your marketing email being marked as spam hurts in more ways than one. It means your email isn’t being viewed by its recipient if they do see it, they think it’s unimportant, and it’s more likely for your domain (or your sender’s if you use an Email Service Provider) to be blacklisted by the recipient’s email service. Meaning future emails could also go to their spam folder, even when it’s not email marketing. Today we’re going to talk about the factors that cause emails to go to spam: the spam filters. Let’s go over what they are, how they work, and what good practices and procedures you can use to avoid getting dropped into the spam folder.

Unsolicited Bulk Email: What Spam Filters Block

Spam is slang for a problem that has been around for as long as the internet: unsolicited bulk emails. The technical terminology actually does a good job of describing the problem. Unsolicited – recipient didn’t ask to have emails sent to them from this domain; Bulk – these emails are sent out en masse; and emails – this communication is through and regulated by email platforms, those whose filters spell the difference between a received email and one held by spam filters or even hard bounced.

How Email Spam Filters Work

Email spam filters are actually a series of multiple filters that analyze any email as it comes in, looking at the various parts and comparing them to multiple lists and rules. While it differs between the individual emails (Outlook, Gmail, etc.), below are the most common filters.

  • Blacklists: IP addresses and domains that send out a lot of emails that are flagged as spam can become blacklisted with all future emails going to spam or being hard bounced. These affect sender and IP reputation, which in turn can affect email acceptance, a reason we and other Email Service Provider (ESPs) work so hard to reduce spam and hard bounces.
  • Header Filters: The header (the “From” email on the recipient’s side) needs to be a valid email. Email platforms have standards compliance and use authentication to validate the email is genuine.
  • Content Filters: All content, from the subject line to the footer, is weighted and measured. Spam filters look for flagged keywords, phrases, and even things like excessive punctuation (!!!).
  • User and Permission Filters: Individual users can also apply additional filters to their spam, either by flagging emails as spam, setting permission filters, or adding rules.

How to Avoid Spam Filters

The best way to avoid spam filters is to use the golden rule: treat others the way you want to be treated. You want to create emails that are useful and engaging to the user and avoid all the tropes of bad spammy email, from overused and leading subject lines to bloated content loaded down with keywords. Below are the top 5 ways to avoid spam filters:

  1. Have Permission to Send: Respectfully using personal data is important, especially in this post-GDPR email Never buy lists and always explicitly ask your users for permission to send them emails.
  2. Be Kind to Your Subject Line: Avoid using exclamation marks, all caps, and things like emojis in your subject lines.
  3. Avoid Common Spam Phrases: Try and get away from sounding overly salesperson-like in your emails. Claims of “Making Money Online TODAY!” or “Work from home!” are going to land you in hot water. Also avoid comments that sound like phishing, such as “Your Account.”
  4. Have Good Content: Filling your email with a keyword-soup or a lot of images and no content is something spam filters stop back users don’t want to see it. Work on providing better content and think about how you can send better emails using features like RSS-to-Email.
  5. Work with a Good ESP: The reputation of your sending platform is important. Working is an Email Service Provider not only have great sending reputations, but they also have tools to help you avoid sending spam by catching it and helping you correct it.

Check out more information about how ESPs can help in our other post, Let’s Talk about Spam.

Want to avoid spam and take your email marketing to the next level? It’s time to contact FeedBlitz. Send our team an email at You can also use our chat, check out our Help Forum, or give us a call at 1.877.692.5489. Our Support and Sales Desk is available Monday – Friday from 9 am to 5 pm Eastern.